<?
   session_start();
   $pool = '0123456789';
   $img_width = 80;
   $img_height = 30;
   $str = '';
   for ($i = 0; $i < 4; $i++){
       $str .= substr($pool, mt_rand(0, strlen($pool) -1), 1);
   }
   $string = $str;
   $im = imagecreate($img_width, $img_height);
   $bg_color     = imagecolorallocate($im,103,103,103);
   $font_color   = imagecolorallocate($im,252,252,252);
   $grid_color   = imagecolorallocate($im,31,0,0);
   $border_color = imagecolorallocate ($im, 174, 174, 174);
   imagefill($im,1,1,$bg_color);
   for($i=0; $i<500; $i++){

       $rand1 = rand(0,$img_width);
       $rand2 = rand(0,$img_height);
       imageline($im, $rand1, $rand2, $rand1, $rand2, $grid_color);

   }
   $x = rand(5, $img_width/(7/2));
   imagerectangle($im, 0, 0, $img_width-1, $img_height-1, $border_color);
   for($a=0; $a < 4; $a++){

       imagestring($im, 5, $x, rand(6 , $img_height/5), substr($string, $a, 1), $font_color);
       $x += (5*2); #odstęp
   }
   $_SESSION['string'] = $string;
   header("Content-type: image/gif");
   imagegif($im);
   imagedestroy($im);

?>