写了一个PHP随机问答验证码，安全性个人认为仍是不错的

<?php  //$i为要写在画板的位置，$num为要写的文字  function rand4string($i,$num){      global $image_width,$image_height,$img;      $font=mt_rand(3,5);      $x=mt_rand(1,8)+$image_width*$i/6;      $y=mt_rand(1,$image_height/4);       $color=imagecolorallocate($img,mt_rand(0,100),mt_rand(0,150),mt_rand(0,200));      imagestring($img,$font,$x,$y,$num,$color);  }  session_start();  header("content-type:image/png");  $image_width=80;  $image_height=18;  $img = imagecreate($image_width,$image_height);  imagecolorallocate($img,255,255,255);  $random4symbol = array("+","-","x");    //三个符号进行循环  $result = 0;  $symbol = $random4symbol[mt_rand(0,count($random4symbol)-1)];  $num1 = mt_rand(1,9);  rand4string(0,$num1);  $num2 = mt_rand(1,9);  switch ($symbol){      case "+":          $result = $num1 + $num2;          rand4string(1,"+");          break;      case "-":          $result = $num1 - $num2;          rand4string(1,"-");          break;      case "x":          $result = $num1 * $num2;          rand4string(1,"x");          break;  }  rand4string(2,$num2);  rand4string(3,"=");  rand4string(4,"?");  $_SESSION['code'] = $result;  imagepng($img);  imagedestroy($img);  ?>