> 16;//233   15326445/2^16 = 233.862991333
//echo decbin($rgb >> 16);//11101001  将$rgb的二进制值向右移位16，超出部分去掉得到
//$r = ($rgb >> 16) & 0xFF;
//$g = ($rgb >> 8) & 0xFF;
//$b = $rgb & 0xFF;



function average($img) {
    $w = imagesx($img);
    $h = imagesy($img);
    $r = $g = $b = 0;
    for($y = 0; $y < $h; $y++) {
        for($x = 0; $x < $w; $x++) {

//颜色表示和位操作  http://www.cnblogs.com/mengdd/p/3254292.html
            $rgb = imagecolorat($img, $x, $y);
            $r += $rgb >> 16;
            $g += $rgb >> 8 & 255;
            $b += $rgb & 255;
        }
    }
    $pxls = $w * $h;
    $r = dechex(round($r / $pxls));
    $g = dechex(round($g / $pxls));
    $b = dechex(round($b / $pxls));
    if(strlen($r) < 2) {
        $r = 0 . $r;
    }
    if(strlen($g) < 2) {
        $g = 0 . $g;
    }
    if(strlen($b) < 2) {
        $b = 0 . $b;
    }
    return "#" . $r . $g . $b;
}


print_r(average($im));

这段代码

$r += $rgb >> 16;
$g += $rgb >> 8 & 255;
$b += $rgb & 255;
          

不明白的地方是为什么移位后还要位与255操作