str_replace()的语法：
str_replace mixed $search ,mixed $replace, mixed $subject [, int &$count ] ) : mixed
1.search ,replace都是字符串
<?php
$bodytag = str_replace("%body%", "black", "<body text='%body%'>");
echo $bodytag; //输出：<body text='black'>
?>2.search是数组 ,replace是字符串
<?php
$vowels = array("a", "e", "i", "o", "u", "A", "E", "I", "O", "U");
$onlyconsonants = str_replace($vowels, "", "Hello World of PHP");
echo $onlyconsonants;// 输出： Hll Wrld f PHP
?>3.search ,replace都是数组
<?php
$phrase = "You should eat fruits, vegetables, and fiber every day.";
$healthy = array("fruits", "vegetables", "fiber");
$yummy = array("pizza", "beer", "ice cream");
$newphrase = str_replace($healthy, $yummy, $phrase);
//输出：You should eat pizza, beer, and ice cream every day
?>4.含有第三个参数
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<?php
$str = str_replace("ll", "", "good golly miss molly!", $count);
echo $count;// 赋值: 2
?>ps: search ,replace的第一组替换会影响到第二组，如下
$search = array('A', 'B', 'C', 'D', 'E');
$replace = array('B', 'C', 'D', 'E', 'F');
$subject = 'A';
echo str_replace($search, $replace, $subject)."<br/>";
// 输出 F ，因为 A 被 B 替换，B 又被 C 替换，以此类推...
// 由于从左到右依次替换，最终 E 被 F 替换 
