php生成图片验证码功能示例都有什么？

<?php
/**
 * Created by JetBrains PhpStorm.
 * User: lee
 * To change this template use File | Settings | File Templates.
 */
header("content-type:image/png");
$validateLength=4;
$strToDraw="";
$chars=[
 "0","1","2","3","4",
 "5","6","7","8","9",
 "a","b","c","d","e","f","g",
 "h","i","j","k","l","m","n",
 "o","p","q","r","s","t",
 "u","v","w","x","y","z",
 "A","B","C","D","E","F","G",
 "H","I","J","K","L","M","N",
 "O","P","Q","R","S","T",
 "U","V","W","X","Y","Z"
];
$imgW=80;
$imgH=25;
$imgRes=imagecreate($imgW,$imgH);
$imgColor=imagecolorallocate($imgRes,255,255,100);
$color=imagecolorallocate($imgRes,0,0,0);
for($i=0;$i<$validateLength;$i++){
 $rand=rand(1,58);
 $strToDraw=$strToDraw." ".$chars[$rand];
}
imagestring($imgRes,5,0,5,$strToDraw,$color);
for($i=0;$i<100;$i++){
 imagesetpixel($imgRes,rand(0,$imgW),rand(0,$imgH),$color);
}
imagepng($imgRes);
imagedestroy($imgRes);