PHP验证码3

查看效果:php验证码3
代码如下:
<?php
session_start();
function randString($length)
{$chars = array( 'a', 'A', 'b', 'B', 'c', 'C', 'd', 'D', 'e', 'E', 'f', 'F', 'g', 'G', 'h', 'H', 'i', 'I', 'j', 'J',  'k', 'K', 'l', 'L', 'm', 'M', 'n', 'N', 'o', 'O', 'p', 'P', 'q', 'Q', 'r', 'R', 's', 'S', 't', 'T',  'u', 'U', 'v', 'V', 'w', 'W', 'x', 'X', 'y', 'Y', 'z', 'Z', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0');
$max_chars = count($chars) - 1;
srand((double) microtime()*1000000);
$rand_str = '';
for($i = 0; $i < $length; $i++) {
$rand_str = ( $i == 0 ) ? $chars[rand(0, $max_chars)] : $rand_str . $chars[rand(0, $max_chars)];
}
return $rand_str;
}
$str = randString(4);
$mode = (isset($_GET['mode'])) ? $_GET['mode'] : 'login';
if ($mode == 'register') {
$_SESSION['register_verifycode']      = $str;
$_SESSION['register_verifycode_time'] = time();
} else {
$_SESSION['login_verifycode']      = $str;
$_SESSION['login_verifycode_time'] = time();
}
$img = ImageCreate(50, 20);
$fg  = ImageColorAllocate($img, 0, 0, 0);
$bg  = ImageColorAllocate($img, 120, 240, 216);
ImageFilledRectangle($img, 0, 0, 50, 20, $bg);
ImageString($img, 4, 10, 2, $str, $fg);
header("Content-Type: image/png");
ImagePNG($img);
?>