PHP自定义大小验证码的方法详解

<?php
function vcode($num=4,$size=20, $width=0,$height=0){
        !$width && $width = $num*$size*4/5+5;
        !$height && $height = $size + 10;
        // 去掉了 0 1 o l 等
            $str = "23456789abcdefghijkmnpqrstuvwxyzabcdefghijklmnpqrstuvw";
            $code = '';
            for ($i=0; $i<$num; $i++){
                    $code.= $str[mt_rand(0, strlen($str)-1)];
            }
            // 画图像
            $im = imagecreatetruecolor($width,$height);
            // 定义要用到的颜色
            $back_color = imagecolorallocate($im, 235, 236, 237);
            $boer_color = imagecolorallocate($im, 118, 151, 199);
            $text_color = imagecolorallocate($im, mt_rand(0,200), mt_rand(0,120), mt_rand(0,120));

            // 画背景
        imagefilledrectangle($im,0,0,$width,$height,$back_color);
            // 画边框
            imagerectangle($im,0,0,$width-1,$height-1,$boer_color);
            // 画干扰线
            for($i=0;$i<5;$i++){
                    $font_color = imagecolorallocate($im, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255));
                imagearc($im,mt_rand(-$width,$width),mt_rand(-$height,$height),mt_rand(30,$width*2),mt_rand(20,$height*2),mt_rand(0,360),mt_rand(0,360),$font_color);
                }
        // 画干扰点
        for($i=0;$i<50;$i++){
                $font_color = imagecolorallocate($im, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255));
                imagesetpixel($im,mt_rand(0,$width),mt_rand(0,$height),$font_color);
        }
        // 画验证码
        @imagefttext($im, $size , 0, 5, $size+3, $text_color, 'c://windows//fonts//simsun.ttc',$code);
        header("cache-control: max-age=1, s-maxage=1, no-cache, must-revalidate");
        header("content-type: image/png");
        imagepng($im);
        imagedestroy($im);
}
?>