PHPプロット関数を利用した簡単な検証コード機能を実現する方法

<?php
//===================================》》           

//1.    4    
$checkCode="";
for ($i=0;$i<4;$i++){
$checkCode.=dechex(rand(1, 15));// decheck()          ,           
}

//2.   
session_start();
$_SESSION['checkCode']=$checkCode;

//3.    
$image1=imagecreatetruecolor(100, 30);

//    ,  20   
for ($j=0;$j<30;$j++){
imagearc($image1, rand(0, 100), rand(0, 30), rand(0, 100), rand(0, 30), rand(0, 360), rand(0, 360), imagecolorallocate($image1, rand(0, 155), rand(0, 255), rand(0, 255)));
}

//3.      ,      
$white=imagecolorallocate($image1, 255, 255, 255);
imagestring($image1, rand(2, 5), rand(5, 70), rand(2, 15), $checkCode, $white);

//5.       
header("content-type:image/png");
imagepng($image1);

//6.    
imagedestroy($image1);

      ：<img src="index.php" onclick="this.src='index.php?a=+random()'">