php:验证码

<?php$checkCode = "";for ($i = 0; $i < 4; $i++) {    $checkCode .= dechex(rand(1, 15));}session_start();$_SESSION['checkcode'] = $checkCode;$image1 = imagecreatetruecolor(100, 40);$red = imagecolorallocate($image1, 255, 0, 0);//干扰线；for ($i = 0;$i<14;$i++){    imageline($image1,rand(0,50),rand(0,20),rand(0,100),        rand(0,40),imagecolorallocate($image1,        rand(0,255),rand(0,255),rand(0,255)));}imagestring($image1,5,rand(0,20),rand(0,10),$checkCode,$red);header("content-type:image/png");imagepng($image1);imagedestroy($image1);

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请输入验证码：<img src="validate.php"    onclick="this.src='validate.php?aa='+Math.random()">