PHP写的验证码

<?php
header('content-type:image/png');
$w = 200;
$h = 50;
$i = imagecreatetruecolor($w,$h);
$bgc = imagecolorallocate($i,251,251,251);
imagefill($i,0,0,$bgc);




function getStr($len = 4){
    $str = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    $s = '';
    for($i=0;$i<$len;$i++){
        $s.=$str{mt_rand(0,strlen($str)-1)};
    }
    return $s;
}
//$c = imagecolorallocate($i,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));
//imagettftext($i,20,mt_rand(0,10),10,35,$c,'font/h.ttf',getStr(10));






$s = getStr(6);
for($n=0;$n<strlen($s);$n++){
    $c = imagecolorallocate($i,mt_rand(0,155),mt_rand(0,155),mt_rand(0,155));
    imagettftext($i,mt_rand(15,30),mt_rand(-15,30),10+($n*18),mt_rand(25,35),$c,'font/h.ttf',$s{$n});
}




$s = getStr(20);
for($n=0;$n<strlen($s);$n++){
    $c = imagecolorallocatealpha($i,mt_rand(0,255),mt_rand(0,255),mt_rand(0,255),50);
    imagettftext($i,mt_rand(5,10),mt_rand(-15,30),mt_rand(0,$w),mt_rand(0,$h),$c,'font/h.ttf',$s{$n});
}




imagepng($i);
imagedestroy($i);