Example of verification code for PHP generation addition and subtraction algorithm

<?php 
namespace mobile\components;
/**
  * @author fenghuo
  *
  * Renovated addition and subtraction verification class
  * Examples of use VerifyCode::get(1,2);
  * VerifyCode::check($code);
  */ 
class VerifyCode 
{ 
  /**
    * php verification code
    */ 
  public static function get($one,$two,$prefix = '', $font_size = 28) 
  { 
    //File header...    ob_get_clean();
    header("Content-type: image/png;charset=utf-8;"); 
    //Create true color white paper    $width      = $font_size*5; 
    $height      = $font_size+1; 
    $im        = @imagecreatetruecolor($width, $height) or die("Episode creation failed"); 
    //Get background color    $background_color = imagecolorallocate($im, 255, 255, 255); 
    //Fill background color    imagefill($im, 0, 0, $background_color); 
    //Get border color    $border_color   = imagecolorallocate($im, 200, 200, 200); 
    //Draw rectangles, border colors 200,200,200,200    imagerectangle($im,0,0,$width - 1, $height - 1,$border_color); 
    // Show off the background line by line, use 1 or 0 in full screen    for($i = 2;$i < $height - 2;$i++) { 
      //Get random pale colors      $line_color = imagecolorallocate($im, rand(200,255), rand(200,255), rand(200,255)); 
      //Draw lines      imageline($im, 2, $i, $width - 1, $i, $line_color); 
    } 
    //Set the printed text    $firstNum = $one; 
    $secondNum = $two; 
    $actionStr = $firstNum > $secondNum ? '-' : '+'; 
    //Get the first random text    $imstr[0]["s"] = $firstNum; 
    $imstr[0]["x"] = rand(2, 5); 
    $imstr[0]["y"] = rand(1, 4); 
    //Get the second random text    $imstr[1]["s"] = $actionStr; 
    $imstr[1]["x"] = $imstr[0]["x"] + $font_size - 1 + rand(0, 1); 
    $imstr[1]["y"] = rand(1,5); 
    //Get the third random text    $imstr[2]["s"] = $secondNum; 
    $imstr[2]["x"] = $imstr[1]["x"] + $font_size - 1 + rand(0, 1); 
    $imstr[2]["y"] = rand(1, 5); 
    //Get the third random text    $imstr[3]["s"] = '='; 
    $imstr[3]["x"] = $imstr[2]["x"] + $font_size - 1 + rand(0, 1); 
    $imstr[3]["y"] = 3; 
    //Get the third random text    $imstr[4]["s"] = '?'; 
    $imstr[4]["x"] = $imstr[3]["x"] + $font_size - 1 + rand(0, 1); 
    $imstr[4]["y"] = 3; 
    //Word    $text = ''; 
    //Write random string    for($i = 0; $i < 5; $i++) { 
      //Get random darker colors      $text_color = imagecolorallocate($im, rand(50, 180), rand(50, 180), rand(50, 180)); 
      $text .= $imstr[$i]["s"]; 
      //Draw text      imagechar($im, $font_size, $imstr[$i]["x"], $imstr[$i]["y"], $imstr[$i]["s"], $text_color); 
    } 
    session_start();
    $_SESSION[$prefix.'verifycode'] = $firstNum > $secondNum ? ($firstNum - $secondNum) : ($firstNum + $secondNum); 
    //Show pictures    ImagePng($im); 
    //Destroy the picture    ImageDestroy($im); 
  } 
  public static function check($code) 
  { 
    if(trim($_SESSION[$prefix.'verifycode']) == trim($code)) { 
      return true; 
    } else { 
      return false; 
    } 
  } 
}