位运算符允许对整型数中指定的位进行求值和操作。
| 例子 | 名称 | 结果 |
|---|---|---|
| And（按位与） | 将把 $a 和 $b 中都为 1 的位设为 1。 | |
| Or（按位或） | 将把 $a 和 $b 中任何一个为 1 的位设为 1。 | |
| Xor（按位异或） | 将把 $a 和 $b 中一个为 1 另一个为 0 的位设为 1。 | |
| Not（按位取反） | 将 $a 中为 0 的位设为 1，反之亦然。 | |
| Shift left（左移） | 将 $a 中的位向左移动 $b 次（每一次移动都表示“乘以 2”）。 | |
| Shift right（右移） | 将 $a 中的位向右移动 $b 次（每一次移动都表示“除以 2”）。 |
位移在 PHP 中是数学运算。向任何方向移出去的位都被丢弃。左移时右侧以零填充，符号位被移走意味着正负号不被保留。右移时左侧以符号位填充，意味着正负号被保留。
要用括号确保想要的优先级。例如 $a & $b == true 先进行比较再进行按位与；而 ($a & $b) == true 则先进行按位与再进行比较。
要注意数据类型的转换。如果左右参数都是字符串，则位运算符将对字符的 ASCII 值进行操作。
PHP 的 ini 设定 error_reporting 使用了按位的值，
提供了关闭某个位的真实例子。要显示除了提示级别
之外的所有错误，php.ini 中是这样用的：
E_ALL & ~E_NOTICE 具体运作方式是先取得 E_ALL 的值： 00000000000000000111011111111111 再取得 E_NOTICE 的值： 00000000000000000000000000001000 然后通过 ~ 将其取反： 11111111111111111111111111110111 最后再用按位与 AND（&）得到两个值中都设定了（为 1）的位： 00000000000000000111011111110111
另外一个方法是用按位异或 XOR（^）来取得只在 其中一个值中设定了的位：E_ALL ^ E_NOTICE
error_reporting 也可用来演示怎样置位。只显示错误和可恢复
错误的方法是：
E_ERROR | E_RECOVERABLE_ERROR 也就是将 E_ERROR 00000000000000000000000000000001 和 E_RECOVERABLE_ERROR 00000000000000000001000000000000 用按位或 OR（|）运算符来取得在任何一个值中被置位的结果： 00000000000000000001000000000001
Example #1 整数的 AND，OR 和 XOR 位运算符
* Ignore the top section,
* it is just formatting to make output clearer.
*/
$format = '(%1$2d = %1$04b) = (%2$2d = %2$04b)'
. ' %3$s (%4$2d = %4$04b)' . "\n";
echo --------- --------- -- ---------
result value op test
--------- --------- -- ---------
EOH;
/*
* Here are the examples.
*/
$values = array(0, 1, 2, 4, 8);
$test = 1 + 4;
echo "\n Bitwise AND \n";
foreach ($values as $value) {
$result = $value & $test;
printf($format, $result, $value, '&', $test);
}
echo "\n Bitwise Inclusive OR \n";
foreach ($values as $value) {
$result = $value | $test;
printf($format, $result, $value, '|', $test);
}
echo "\n Bitwise Exclusive OR (XOR) \n";
foreach ($values as $value) {
$result = $value ^ $test;
printf($format, $result, $value, '^', $test);
}
?>
立即学习“PHP免费学习笔记（深入）”；
以上例程会输出：
--------- --------- -- --------- result value op test --------- --------- -- --------- Bitwise AND ( 0 = 0000) = ( 0 = 0000) & ( 5 = 0101) ( 1 = 0001) = ( 1 = 0001) & ( 5 = 0101) ( 0 = 0000) = ( 2 = 0010) & ( 5 = 0101) ( 4 = 0100) = ( 4 = 0100) & ( 5 = 0101) ( 0 = 0000) = ( 8 = 1000) & ( 5 = 0101) Bitwise Inclusive OR ( 5 = 0101) = ( 0 = 0000) | ( 5 = 0101) ( 5 = 0101) = ( 1 = 0001) | ( 5 = 0101) ( 7 = 0111) = ( 2 = 0010) | ( 5 = 0101) ( 5 = 0101) = ( 4 = 0100) | ( 5 = 0101) (13 = 1101) = ( 8 = 1000) | ( 5 = 0101) Bitwise Exclusive OR (XOR) ( 5 = 0101) = ( 0 = 0000) ^ ( 5 = 0101) ( 4 = 0100) = ( 1 = 0001) ^ ( 5 = 0101) ( 7 = 0111) = ( 2 = 0010) ^ ( 5 = 0101) ( 1 = 0001) = ( 4 = 0100) ^ ( 5 = 0101) (13 = 1101) = ( 8 = 1000) ^ ( 5 = 0101)
Example #2 字符串的 XOR 运算符
echo "12" ^ "9"; // Outputs the Backspace character (ascii 8)
// ('1' (ascii 49)) ^ ('9' (ascii 57)) = #8
echo "hallo" ^ "hello"; // Outputs the ascii values #0 #4 #0 #0 #0
// 'a' ^ 'e' = #4
echo 2 ^ "3"; // Outputs 1
// 2 ^ ((int)"3") == 1
echo "2" ^ 3; // Outputs 1
// ((int)"2") ^ 3 == 1
?>
Example #3 整数的位移
* Here are the examples.
*/
echo "\n--- BIT SHIFT RIGHT ON POSITIVE INTEGERS ---\n";
$val = 4;
$places = 1;
$res = $val >> $places;
p($res, $val, '>>', $places, 'copy of sign bit shifted into left side');
$val = 4;
$places = 2;
$res = $val >> $places;
p($res, $val, '>>', $places);
$val = 4;
$places = 3;
$res = $val >> $places;
p($res, $val, '>>', $places, 'bits shift out right side');
$val = 4;
$places = 4;
$res = $val >> $places;
p($res, $val, '>>', $places, 'same result as above; can not shift beyond 0');
echo "\n--- BIT SHIFT RIGHT ON NEGATIVE INTEGERS ---\n";
$val = -4;
$places = 1;
$res = $val >> $places;
p($res, $val, '>>', $places, 'copy of sign bit shifted into left side');
$val = -4;
$places = 2;
$res = $val >> $places;
p($res, $val, '>>', $places, 'bits shift out right side');
$val = -4;
$places = 3;
$res = $val >> $places;
p($res, $val, '>>', $places, 'same result as above; can not shift beyond -1');
echo "\n--- BIT SHIFT LEFT ON POSITIVE INTEGERS ---\n";
$val = 4;
$places = 1;
$res = $val $places;
p($res, $val, ', $places, 'zeros fill in right side');
$val = 4;
$places = (PHP_INT_SIZE * 8) - 4;
$res = $val $places;
p($res, $val, ', $places);
$val = 4;
$places = (PHP_INT_SIZE * 8) - 3;
$res = $val $places;
p($res, $val, ', $places, 'sign bits get shifted out');
$val = 4;
$places = (PHP_INT_SIZE * 8) - 2;
$res = $val $places;
p($res, $val, ', $places, 'bits shift out left side');
echo "\n--- BIT SHIFT LEFT ON NEGATIVE INTEGERS ---\n";
$val = -4;
$places = 1;
$res = $val $places;
p($res, $val, ', $places, 'zeros fill in right side');
$val = -4;
$places = (PHP_INT_SIZE * 8) - 3;
$res = $val $places;
p($res, $val, ', $places);
$val = -4;
$places = (PHP_INT_SIZE * 8) - 2;
$res = $val $places;
p($res, $val, ', $places, 'bits shift out left side, including sign bit');
/*
* Ignore this bottom section,
* it is just formatting to make output clearer.
*/
function p($res, $val, $op, $places, $note = '') {
$format = '%0' . (PHP_INT_SIZE * 8) . "b\n";
printf("Expression: %d = %d %s %d\n", $res, $val, $op, $places);
echo " Decimal:\n";
printf(" val=%d\n", $val);
printf(" res=%d\n", $res);
echo " Binary:\n";
printf(' val=' . $format, $val);
printf(' res=' . $format, $res);
if ($note) {
echo " NOTE: $note\n";
}
echo "\n";
}
?>
以上例程在 32 位机器上的输出:
--- BIT SHIFT RIGHT ON POSITIVE INTEGERS --- Expression: 2 = 4 >> 1 Decimal: val=4 res=2 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000010 NOTE: copy of sign bit shifted into left side Expression: 1 = 4 >> 2 Decimal: val=4 res=1 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000001 Expression: 0 = 4 >> 3 Decimal: val=4 res=0 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000000 NOTE: bits shift out right side Expression: 0 = 4 >> 4 Decimal: val=4 res=0 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000000 NOTE: same result as above; can not shift beyond 0 --- BIT SHIFT RIGHT ON NEGATIVE INTEGERS --- Expression: -2 = -4 >> 1 Decimal: val=-4 res=-2 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111110 NOTE: copy of sign bit shifted into left side Expression: -1 = -4 >> 2 Decimal: val=-4 res=-1 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111111 NOTE: bits shift out right side Expression: -1 = -4 >> 3 Decimal: val=-4 res=-1 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111111 NOTE: same result as above; can not shift beyond -1 --- BIT SHIFT LEFT ON POSITIVE INTEGERS --- Expression: 8 = 4 << 1 Decimal: val=4 res=8 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000001000 NOTE: zeros fill in right side Expression: 1073741824 = 4 << 28 Decimal: val=4 res=1073741824 Binary: val=00000000000000000000000000000100 res=01000000000000000000000000000000 Expression: -2147483648 = 4 << 29 Decimal: val=4 res=-2147483648 Binary: val=00000000000000000000000000000100 res=10000000000000000000000000000000 NOTE: sign bits get shifted out Expression: 0 = 4 << 30 Decimal: val=4 res=0 Binary: val=00000000000000000000000000000100 res=00000000000000000000000000000000 NOTE: bits shift out left side --- BIT SHIFT LEFT ON NEGATIVE INTEGERS --- Expression: -8 = -4 << 1 Decimal: val=-4 res=-8 Binary: val=11111111111111111111111111111100 res=11111111111111111111111111111000 NOTE: zeros fill in right side Expression: -2147483648 = -4 << 29 Decimal: val=-4 res=-2147483648 Binary: val=11111111111111111111111111111100 res=10000000000000000000000000000000 Expression: 0 = -4 << 30 Decimal: val=-4 res=0 Binary: val=11111111111111111111111111111100 res=00000000000000000000000000000000 NOTE: bits shift out left side, including sign bit
以上例程在 64 位机器上的输出:
--- BIT SHIFT RIGHT ON POSITIVE INTEGERS --- Expression: 2 = 4 >> 1 Decimal: val=4 res=2 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000010 NOTE: copy of sign bit shifted into left side Expression: 1 = 4 >> 2 Decimal: val=4 res=1 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000001 Expression: 0 = 4 >> 3 Decimal: val=4 res=0 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: bits shift out right side Expression: 0 = 4 >> 4 Decimal: val=4 res=0 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: same result as above; can not shift beyond 0 --- BIT SHIFT RIGHT ON NEGATIVE INTEGERS --- Expression: -2 = -4 >> 1 Decimal: val=-4 res=-2 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111110 NOTE: copy of sign bit shifted into left side Expression: -1 = -4 >> 2 Decimal: val=-4 res=-1 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111111 NOTE: bits shift out right side Expression: -1 = -4 >> 3 Decimal: val=-4 res=-1 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111111 NOTE: same result as above; can not shift beyond -1 --- BIT SHIFT LEFT ON POSITIVE INTEGERS --- Expression: 8 = 4 << 1 Decimal: val=4 res=8 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000001000 NOTE: zeros fill in right side Expression: 4611686018427387904 = 4 << 60 Decimal: val=4 res=4611686018427387904 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0100000000000000000000000000000000000000000000000000000000000000 Expression: -9223372036854775808 = 4 << 61 Decimal: val=4 res=-9223372036854775808 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=1000000000000000000000000000000000000000000000000000000000000000 NOTE: sign bits get shifted out Expression: 0 = 4 << 62 Decimal: val=4 res=0 Binary: val=0000000000000000000000000000000000000000000000000000000000000100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: bits shift out left side --- BIT SHIFT LEFT ON NEGATIVE INTEGERS --- Expression: -8 = -4 << 1 Decimal: val=-4 res=-8 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1111111111111111111111111111111111111111111111111111111111111000 NOTE: zeros fill in right side Expression: -9223372036854775808 = -4 << 61 Decimal: val=-4 res=-9223372036854775808 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=1000000000000000000000000000000000000000000000000000000000000000 Expression: 0 = -4 << 62 Decimal: val=-4 res=0 Binary: val=1111111111111111111111111111111111111111111111111111111111111100 res=0000000000000000000000000000000000000000000000000000000000000000 NOTE: bits shift out left side, including sign bit
Warning
不要在 32 位系统下向右移超过 32 位。不要在结果可能超过 32 的情况下左移。使用 gmp 扩展对超出 PHP_INT_MAX 的数值来进行位操作。
参见 pack()，unpack()，gmp_and()，gmp_or()，gmp_xor()，gmp_testbit() 和 gmp_clrbit()。
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a note
User Contributed Notes 43 notes
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SmartB2B 是一款基于PHP、MySQL、Smarty的B2B行业电子商务网站管理系统，系统提供了供求模型、企业模型、产品模型、人才招聘模型、资讯模型等模块，适用于想在行业里取得领先地位的企业快速假设B2B网站，可以运行于Linux与Windows等多重服务器环境，安装方便，使用灵活。 系统使用当前流行的PHP语言开发，以MySQL为数据库，采用B/S架构，MVC模式开发。融入了模型化、模板
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Sometimes I need a custom PHP Object that holds several boolean TRUE or FALSE values. I could easily include a variable for each of them, but as always, code has a way to get unweildy pretty fast. A more intelligent approach always seems to be the answer, even
if it seems to be overkill at first.
I start with an abstract base class which will hold a single integer variable called $flags. This simple integer can hold 32 TRUE or FALSE boolean values. Another thing to consider is to just set certain BIT values without disturbing any of the other BITS --
so included in the class definition is the setFlag($flag, $value) function, which will set only the chosen bit. Here's the abstract base class definition:
# BitwiseFlag.php
abstract class BitwiseFlag
{
protected $flags;
/*
* Note: these functions are protected to prevent outside code
* from falsely setting BITS. See how the extending class 'User'
* handles this.
*
*/
protected function isFlagSet($flag)
{
return (($this->flags & $flag)
== $flag);
}
protected function setFlag($flag, $value)
{
if($value)
{
$this->flags |= $flag;
}
else
{
$this->flags &=
~$flag;
}
}
}
?>
The class above is abstract and cannot be instantiated, so an extension is required. Below is a simple extension called User -- which is severely truncated for clarity. Notice I am defining const variables AND methods to use them.
# User.php
require('BitwiseFlag.php');
class User extends BitwiseFlag
{
const FLAG_REGISTERED = 1; //
BIT #1 of $flags has the value 1
const FLAG_ACTIVE = 2;
// BIT #2 of $flags has the value 2
const FLAG_MEMBER = 4;
// BIT #3 of $flags has the value 4
const FLAG_ADMIN = 8;
// BIT #4 of $flags has the value 8
public function isRegistered(){
return $this->isFlagSet(self::FLAG_REGISTERED);
}
public function isActive(){
return $this->isFlagSet(self::FLAG_ACTIVE);
}
public function isMember(){
return $this->isFlagSet(self::FLAG_MEMBER);
}
public function isAdmin(){
return $this->isFlagSet(self::FLAG_ADMIN);
}
public function setRegistered($value){
$this->setFlag(self::FLAG_REGISTERED, $value);
}
public function setActive($value){
$this->setFlag(self::FLAG_ACTIVE, $value);
}
public function setMember($value){
$this->setFlag(self::FLAG_MEMBER, $value);
}
public function setAdmin($value){
$this->setFlag(self::FLAG_ADMIN, $value);
}
public function __toString(){
return 'User [' .
($this->isRegistered()
? 'REGISTERED' : '')
.
($this->isActive()
? ' ACTIVE' : '')
.
($this->isMember()
? ' MEMBER' : '')
.
($this->isAdmin()
? ' ADMIN' : '')
.
']';
}
}
?>
This seems like a lot of work, but we have addressed many issues, for example, using and maintaining the code is easy, and the getting and setting of flag values make sense. With the User class, you can now see how easy and intuitive bitwise flag operations
become.
require('User.php')
$user = new User();
$user->setRegistered(true);
$user->setActive(true);
$user->setMember(true);
$user->setAdmin(true);
echo $user; //
outputs: User [REGISTERED ACTIVE MEMBER ADMIN]
?>
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Actually, this is not an issue with the implementation of XOR, but a lot more to do with the lose-typing policy that PHP adopts.
Freely switching between int and float is good for most cases, but problems happen when your value is near the word size of your machine. Which is to say, 32-bit machines will encounter problems with values that hover around 0x80000000 - primarily because PHP
does not support unsigned integers.
using bindec/decbin would address this issue as a work-around to do unsigned-int xor, but here's the real picture (i'm not claiming that this code will perform better, but this would be a better pedagogical code):
function unsigned_xor32 ($a, $b)
{
$a1 = $a & 0x7FFF0000;
$a2 = $a & 0x0000FFFF;
$a3 = $a & 0x80000000;
$b1 = $b & 0x7FFF0000;
$b2 = $b & 0x0000FFFF;
$b3 = $b & 0x80000000;
$c = ($a3 != $b3)
? 0x80000000 : 0;
return (($a1 ^ $b1)
|($a2 ^ $b2))
+ $c;
}
$x = 3851235679;
$y = 43814;
echo "
This is the value we want";
echo "
3851262585";
echo "
The result of a native xor operation on integer values is treated as a signed integer";
echo "
".($x ^ $y);
echo "
We therefore perform the MSB separately";
echo "
".unsigned_xor32($x, $y);
?>
This is really foundation stuff, but for those of you who missed this in college, there seems to be something on 2's complement here:
http://www.evergreen.edu/biophysics/technotes/program/2s_comp.htm
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protected function flipBin($number) {
$bin = str_pad(base_convert($number, 10, 2), 32, 0, STR_PAD_LEFT);
for ($i = 0; $i switch ($bin{$i}) {
case '0' :
$bin{$i} = '1';
break;
case '1' :
$bin{$i} = '0';
break;
}
}
return bindec($bin);
}
the benefit is, it works with numbers greater MAX_INT
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$b = 10;
echo $a & $b;
place value 128 64 32 16 8 4 2 1
$a 0 0 0 0 1 0 0 1 =9
$b 0 0 0 0 1 0 1 0 =10
result 8
only bit they share together is the 8 bit. So 8 gets returned.
$a = 36;
$b = 103;
echo $a & $b;
place value 128 64 32 16 8 4 2 1
$a 0 0 1 0 0 1 0 0 =36
$b 0 1 1 0 0 1 1 1 =103
result 32+4 = 36
the only bits these two share together are the bits 32 and 4 which when added together return 36.
$a = 9;
$b = 10;
echo $a | $b;
place value 128 64 32 16 8 4 2 1
$a 0 0 0 0 1 0 0 1 =9
$b 0 0 0 0 1 0 1 0 =10
result 8+2+1 = 11
3 bits set, in the 8, 2, and 1 column.add those up 8+2+1 and you get 11
$a = 9;
$b = 10;
echo $a ^ $b;
place value 128 64 32 16 8 4 2 1
$a 0 0 0 0 1 0 0 1 =9
$b 0 0 0 0 1 0 1 0 =10
result 2+1 = 3
the 2 bit and the 1 bit that they each have set but don't share. Soooo 2+1 = 3
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FE. 0xff >> -2 results in 0x0
and 0xff
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(Note: It took me almost a whole day to get this to work with negative $num values (I couldn't figure out why it sometimes worked and other times didn't), because PHP only has an arithmatic and not a logical bitwise right shift like I am used to. I.e. 0x80000001>>16
will ouputs (in binary) "1111 1111 1111 1111 1000 0000 0000 0000" instead of "0000 0000 0000 0000 1000 0000 0000 0000" like you would expect. To fix this you have to apply the mask (by bitwise &) equal to 0x7FFFFFFF right shifted one less than the offset you
are shifting by.)
function circular_shift($num,$offset)
{ //Do a nondestructive circular bitwise shift, if offset positive shift left, if negative shift right
$num=(int)$num;
$mask=0x7fffffff; //Mask
to cater for the fact that PHP only does arithmatic right shifts and not logical i.e. PHP doesn't give expected output when right shifting negative values
if ($offset>0)
{
$num=($num$offset%32)
| (($num>>(32-$offset%32))
& ($mask>>(31-$offset%32)));
}
elseif ($offset0){
$offset=abs($offset);
$num=(($num>>$offset%32)
& ($mask>>(-1+$offset%32)))
| ($num32-$offset%32));
}
return $num;
}
?>
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function bitxor($str, $key)
{
$xorWidth = PHP_INT_SIZE*8;
// split
$o1 = str_split($str, $xorWidth);
$o2 = str_split(str_pad('', strlen($str), $key), $xorWidth);
$res = '';
$runs = count($o1);
for($i=0;$i$runs;$i++)
$res .= decbin(bindec($o1[$i])
^ bindec($o2[$i]));
return $res;
}
?>
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After attempting to translate a Perl module into PHP, I realized that Perl's implementation of the ^ operator is different than the PHP implementation. By default, Perl treats the variables as floats and PHP as integers. I was able to verify the PHP use of
the operator by stating "use integer;" within the Perl module, which output the exact same result as PHP was using.
The logical decision would be to cast every variable as (float) when using the ^ operator in PHP. However, this will not yield the same results. After about a half hour of banging my head against the wall, I discovered a gem and wrote a function using the
binary-decimal conversions in PHP.
/*
not having much experience with bitwise operations, I cannot tell you that this is the BEST solution, but it certainly is a solution that finally works and always returns the EXACT same result Perl provides.
*/
function binxor($a, $b) {
return bindec(decbin((float)$a ^ (float)$b));
}
//normal PHP code will not yeild the same result as Perl
$result = 3851235679 ^ 43814; //= -443704711
//to get the same result as Perl
$result = binxor(3851235679, 43814); //= 3851262585
//YIPPEE!!!
//to see the differences, try the following
$a = 3851235679 XOR 43814;
$b = 3851235679 ^ 43814; //integer result
$c = (float)3851235679 ^ (float)43814; //same as $b
$d = binxor(3851235679, 43814); //same as Perl!!
echo("A: $a
");
echo("B: $b
");
echo("C: $c
");
echo("D: $d
");
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// The various details a vehicle can have
$hasFourWheels = 1;
$hasTwoWheels = 2;
$hasDoors = 4;
$hasRedColour = 8;
$bike = $hasTwoWheels;
$golfBuggy = $hasFourWheels;
$ford = $hasFourWheels | $hasDoors;
$ferrari = $hasFourWheels | $hasDoors | $hasRedColour;
$isBike = $hasFourWheels & $bike; #
False, because $bike doens't have four wheels
$isGolfBuggy = $hasFourWheels & $golfBuggy; #
True, because $golfBuggy has four wheels
$isFord = $hasFourWheels & $ford; #
True, because $ford $hasFourWheels
?>
And you can apply this to a lot of things, for example, security:
// Security permissions:
$writePost = 1;
$readPost = 2;
$deletePost = 4;
$addUser = 8;
$deleteUser = 16;
// User groups:
$administrator = $writePost | $readPosts | $deletePosts | $addUser | $deleteUser;
$moderator = $readPost | $deletePost | $deleteUser;
$writer = $writePost | $readPost;
$guest = $readPost;
// function to check for permission
function checkPermission($user, $permission)
{
if($user & $permission)
{
return true;
} else {
return false;
}
}
// Now we apply all of this!
if(checkPermission($administrator, $deleteUser))
{
deleteUser("Some
User"); # This is executed because $administrator can $deleteUser
}
?>
Once you get your head around it, it's VERY useful! Just remember to raise each value by the power of two to avoid problems
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// We want to know the red, green and blue values of this color :
$color = 0xFEA946 ;
$red = $color >> 16 ;
$green = ($color & 0x00FF00)
>> 8 ;
$blue = $color & 0x0000FF ;
printf('Red
: %X (%d), Green : %X (%d), Blue : %X (%d)',
$red, $red, $green, $green, $blue, $blue)
;
// Will display...
// Red : FE (254), Green : A9 (169), Blue : 46 (70)
?>
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Note that this function works only with decimal numbers - other types can be converted with pack().
function rotate ( $decimal, $bits)
{
$binary = decbin($decimal);
return (
bindec(substr($binary, $bits).substr($binary, 0, $bits))
);
}
// Rotate 124124 (1111100) to the left with 1 bits
echo rotate(124124, 1);
// = 121 (1111001)
// Rotate 124124 (1111100) to the right with 3 bits
echo rotate(124124,
-3);
// = 79 (1001111)
?>
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echo masksOf(3,10);
function masksOf($n,$bits)
{
$u = pow(2,$bits)-1; //start
value, full flags on.
$masks = array();
while ($u>0)
{
$z = numflags($u);
if ($z==$n) array_push($masks,$u);
$u--;
}
return ($masks);
}
function numflags($n)
{
$k = 0;
while ($n) {
$k += $n & 1;
$n = $n >> 1;
}
return ($k);
// alternately:
// $u = 0;
// for ($k=1;$k// $u+=($n&$k?1:0);
// }
// return ($u);
}
?>
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~2 = -3 because you use the formula ~x = -x - 1 The bitwise complement of a decimal number is the negation of the number minus 1.
NOTE: just using 4 bits here for the examples below but in reality PHP uses 32 bits.
Converting a negative decimal number (ie: -3) into binary takes 3 steps:
1) convert the positive version of the decimal number into binary (ie: 3 = 0011)
2) flips the bits (ie: 0011 becomes 1100)
3) add 1 (ie: 1100 + 0001 = 1101)
You might be wondering how does 1101 = -3. Well PHP uses the method "2's complement" to render negative binary numbers. If the left most bit is a 1 then the binary number is negative and you flip the bits and add 1. If it is 0 then it is positive and you don't
have to do anything. So 0010 would be a positive 2. If it is 1101, it is negative and you flip the bits to get 0010. Add 1 and you get 0011 which equals -3.
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