PHP生成圖形驗證碼（大小寫字母與數字混合）

<?php
    header("content-type:image/png");
    $num = $_GET['num'];
    $imagewidth=60;
    $imageheight=18;
    srand(microtime() * 100000);


$numimage = imagecreate($imagewidth,$imageheight);
imagecolorallocate($numimage,183,180,83);
for($i=0;$i<strlen($num);$i++){
    $font = mt_rand(3,5);
    $x = mt_rand(1,8)+$imagewidth*$i/4;
    $y = mt_rand(1,$imageheight/3);
    $color=imagecolorallocate($numimage,mt_rand(0,50),mt_rand(0,150),mt_rand(0,255));
    imagestring($numimage,$font,$x,$y,$num[$i],$color);
}
imagepng($numimage);
imagedestroy($numimage);

?>
調用方法：
<?php
    session_start();
    include_once 'conn/conn.php';
    $pattern = '1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLOMNOPQRSTUVWXYZ';
    $num = null;
    for($i = 0; $i < 4; $i++) {
        $num .= $pattern{mt_rand(0, 61)};
    }

?>
<div> 驗證碼：
  <input id="lgchk" type="text" />
  <img src="valcode.php?num=<?php echo $num; ?>" width="55" height="18" /> </div></pre>