php图片验证码

一个简单的php图片验证码，显示两个数字，随机进行加法或者减法运算，计算得到的结果。

结果保存到$_SESSION["yzm"]中，当需要验证时可以进行验证。

图片中另外加入了3条干扰线。

<?php
session_start();
$width = 100; 
$height = 25;
$image = imagecreate($width, $height);
$bg = imagecolorallocate($image, 240, 240, 240);
$color = imagecolorallocate($image, 0, 0, 0);
imagefilledrectangle($image, 0, 0, $width, $height, $bg);
$num1 = mt_rand(43, 66);
$num2 = mt_rand(10, 33);
$op = mt_rand(0, 1) ? '+' : "-";
$yzm0 = $num1 . $op . $num2 . "=";
$_SESSION["yzm"] = $op == "+" ? $num1 + $num2 : $num1 - $num2;
for ($i = 0; $i < 3; $i++) {
    imageline($image, mt_rand(0, $width), mt_rand(0, $height), mt_rand(0, $width), mt_rand(0, $height), $color);
}
$font_size = 22; 
$current_x = 5;
foreach (str_split($yzm0) as $char) {
    $random_offset = mt_rand(-5, 5);
    imagestring($image, $font_size, $current_x + $random_offset, 5, $char, $color);
    $current_x += (int)($width / 6);
}
header("Content-Type: image/png");
imagepng($image);
imagedestroy($image);