Captcha verification code class instance implemented by php

<?php
/** Captcha verification code class
 * Date: 2011-02-19
 * Author: fdipzone
 */

class Captcha{ //class start

 private $sname = '';

 public function __construct($sname=''){ // $sname captcha session name
 $this->sname = $sname==''? 'm_captcha' : $sname;
 }

 /** Generate verification code pictures
  * @param int $length Verification code length
  * @param Array $param parameter
  * @return IMG
  */
 public function create($length=4,$param=array()){
 Header("Content-type: image/PNG");
 $authnum = $this->random($length); //Generate verification code characters. 
 $width = isset($param['width'])? $param['width'] : 13; //Text width $height = isset($param['height'])? $param['height'] : 18; //Text height $pnum = isset($param['pnum'])? $param['pnum'] : 100; //The number of interfering pixels $lnum = isset($param['lnum'])? $param['lnum'] : 2; //The number of interference lines
 $this->captcha_session($this->sname,$authnum);  //Write random numbers to session
 $pw = $width*$length+10;
 $ph = $height+6;
  
 $im = imagecreate($pw,$ph);   //imagecreate() Create a new image, a blank image of sizes x_size and y_size. $black = ImageColorAllocate($im, 238,238,238); //Set background color 
 $values = array(
  mt_rand(0,$pw), mt_rand(0,$ph),
  mt_rand(0,$pw), mt_rand(0,$ph),
  mt_rand(0,$pw), mt_rand(0,$ph),
  mt_rand(0,$pw), mt_rand(0,$ph),
  mt_rand(0,$pw), mt_rand(0,$ph),
  mt_rand(0,$pw), mt_rand(0,$ph)
 );
 imagefilledpolygon($im, $values, 6, ImageColorAllocate($im, mt_rand(170,255),mt_rand(200,255),mt_rand(210,255))); //Set the interference polygon base map 
 /* Word */
 for ($i = 0; $i < strlen($authnum); $i++){
  $font = ImageColorAllocate($im, mt_rand(0,50),mt_rand(0,150),mt_rand(0,200));//Set text color  $x = $i/$length * $pw + rand(1, 6); //Set random X coordinates  $y = rand(1, $ph/3);   //Set random Y coordinates  imagestring($im, mt_rand(4,6), $x, $y, substr($authnum,$i,1), $font); 
 }

 /* Add interference pixels */
 for($i=0; $i<$pnum; $i++){
  $dist = ImageColorAllocate($im, mt_rand(0,255),mt_rand(0,255),mt_rand(0,255)); //Set the dot color  imagesetpixel($im, mt_rand(0,$pw) , mt_rand(0,$ph) , $dist); 
 } 

 /* Add interference line */
 for($i=0; $i<$lnum; $i++){
  $dist = ImageColorAllocate($im, mt_rand(50,255),mt_rand(150,255),mt_rand(200,255)); //Set line color  imageline($im,mt_rand(0,$pw),mt_rand(0,$ph),mt_rand(0,$pw),mt_rand(0,$ph),$dist);
 }

 ImagePNG($im); //Export the image to the browser or file in PNG format ImageDestroy($im); //Destroy an image }

 /** Check the verification code
  * @param String $captcha Verification Code
  * @param int $flag After verification is successful 0: session not cleared 1: session cleared
  * @return boolean
  */ 
 public function check($captcha,$flag=1){
 if(empty($captcha)){
  return false;
 }else{
  if(strtoupper($captcha)==$this->captcha_session($this->sname)){ //Detection verification code  if($flag==1){
   $this->captcha_session($this->sname,'');
  }
  return true;
  }else{
  return false;
  }
 }
 }

 /* Generate random number function
  * @param int $length Number of strings that need to be generated randomly
  * @return String
  */
 private function random($length){
 $hash = '';
 $chars = 'ABCDEFGHIJKLMNPQRSTUVWXYZ23456789';
 $max = strlen($chars) - 1;
 for($i = 0; $i < $length; $i++) {
  $hash .= $chars[mt_rand(0, $max)];
 }
 return $hash;
 }

 /** Verification code session processing method
  * @param String $name captcha session name
  * @param String $value
  * @return String
  */
 private function captcha_session($name,$value=null){
 if(isset($value)){
  if($value!==''){
  $_SESSION[$name] = $value;
  }else{
  unset($_SESSION[$name]);
  }
 }else{
  return isset($_SESSION[$name])? $_SESSION[$name] : '';
 }
 }
} // class end
?>

<?php 
  session_start(); 
  require_once(''); 
 
  $obj = new Captcha($sname);   # Create Captcha class object                  # $sname is the session name that saves captcha, can be left blank, and the space is 'm_captcha' 
  $obj->create($length,$param);  #Create Captcha and output pictures                  # $length is Captcha length, can be left blank, default is 4                  /* $param = array(
                       'width' => 13 captcha character width
                       'height' => 18 captcha character height
                       'pnum' => 100 Number of interference points
                       'lnum' => 2 Number of interfering lines
                       )
                       Leave it empty
                   */ 
  $obj->check($captcha,$flag); # Check whether the verification code entered by the user is correct, true or false                  # $captcha is the verification code entered by the user, required                  # $flag can be left blank, default is 1                  #1: Automatically clear captcha session after verification is successful                  # 0: Captcha session is not cleared after successful block verification, used for ajax check?>