php生成验证码

<?php
$rand_bg_file = ‘./captcha/a.jpg‘;//设置背景地址
$img = imagecreatefromjpeg($rand_bg_file);//创建画布
$white = imagecolorallocate($img, 0xff, 0xff, 0xff);//绘制边框
imagerectangle($img, 0, 0, 144, 19, $white);//画一个无填充矩形
$chars = ‘ABCDEFGHJKLMNPQRSTUVWXYZ23456789‘;//32 个字符,随机取出4个来当验证码
$captcha_str = ‘‘;
for($i=1,$strlen=strlen($chars); $i<=4; ++$i) {
	$rand_key = mt_rand(0, $strlen-1);
	$captcha_str .= $chars[$rand_key];
}
@session_start();
$_SESSION[‘captcha_code‘] = $captcha_str;//将验证码值保存到session中
$black = imagecolorallocate($img, 0, 0, 0);//先确定颜色，白黑随机！
if(mt_rand(0, 1) == 1 ) {
	$str_color = $white;
} else {
	$str_color = $black;
}
imagestring($img, 5, 60, 3, $captcha_str, $str_color);//写入文字
header(‘Content-Type: image/jpeg; charset=utf-8‘);//告知浏览器，发送的是jpeg格式的图片
imagejpeg($img);//输出到浏览器端！
imagedestroy($img);//销毁资源