Detail method for PHP custom size captcha

<?php
function vCode($num=4,$size=20, $width=0,$height=0){ 
        !$width && $width = $num*$size*4/5+5; 
        !$height && $height = $size + 10; 
        //  Remove the  0 1 O l  Etc.  
            $str = "23456789abcdefghijkmnpqrstuvwxyzABCDEFGHIJKLMNPQRSTUVW"; 
            $code = ''; 
            for ($i=0; $i<$num; $i++){ 
                    $code.= $str[mt_rand(0, strlen($str)-1)]; 
            } 
            //  Draw the image  
            $im = imagecreatetruecolor($width,$height); 
            //  Define the colors to be used  
            $back_color = imagecolorallocate($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)); 

            //  Draw the background  
        imagefilledrectangle($im,0,0,$width,$height,$back_color); 
            //  Picture frame  
            imagerectangle($im,0,0,$width-1,$height-1,$boer_color); 
            //  Drew interference  
            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); 
                } 
        //  Painting noise points  
        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); 
        } 
        //  Drawing verification code  
        @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); 
} 
?>

Function description and examples:
< ?
// 4 characters, size 20
vCode(4,20);
? >