php implements the addition and subtraction captcha code

<?php
/* Image captcha file, add and subtract calculation method */
class ImageCode{

 private $Jiashu  = 0;        // Plus or minus 
 private $JianShu = 0;        // Plus or minus 
 private $YunSuan = '';       // The operator 
 private $DeShu   = 0;        // It will take 
 private $String  = '';       // String style 
 private $Img;                // Image objects 
 private $Width   = 100;      // Image width 
 private $Height  = 50;       // Picture height 
 private $Ttf     = 'Num.ttf';// The font file 
 private $Session = 'code';   //Session variable 

 private function JiaShu(){
  header('Content-type:image/png');
  $this -> Jiashu  = rand(1, 10);
  $this -> JianShu = rand(1, 10);
  $this -> YunSuan= $this -> Jiashu > $this -> JianShu ? '-' : '+';
  $this -> DeShu   = $this -> Jiashu > $this -> JianShu ? $this -> Jiashu - $this -> JianShu : $this -> Jiashu + $this -> JianShu;
 }

 public function Show( $W = 100, $H = 50, $T = 'Num.ttf', $Code = 'code' ){
  $this -> JiaShu();
  $this -> String = $this -> Jiashu . $this -> YunSuan . $this -> JianShu . '= ? ';
  $this -> Width  = $W;
  $this -> Height = $H;
  $this -> Ttf    = $T;
  $this -> Session= $Code;
  session_start();
  $_SESSION[$this -> Session] = $this -> DeShu;
  $this -> Images();
 }

 private function Images(){
  $this -> Img = imagecreate($this -> Width, $this -> Height);
  $background_color = imagecolorallocate ($this -> Img, 255, 255, 255);
  imagecolortransparent($this -> Img, $background_color);
        imagettftext($this -> Img, 14, 0, 1, 20, imagecolorallocate ($this -> Img, 0, 0, 0), $this -> Ttf, $this -> String );
  $this -> EchoImages();
 }

 private function EchoImages(){
  imagepng($this -> Img);
  imagedestroy($this -> Img);
 }

}
$ImageCode = new ImageCode;
$ImageCode -> Show(130, 35, 'Num.ttf', 'code');