капча

нашёл в книге скрипт капчи.
помогите разобраться с ошибкой.

Warning: imagettftext() [function.imagettftext]: Invalid font filename in Z:\home\localhost\www\1.php on line 31

PHP:

1.  
2. $w=300;
3. $h=50;
4. $gfx=imagecreatetruecolor($w, $h);
5.  
6. imageantialias($gfx, true);
7.  
8. $white = imagecolorallocate($gfx,255, 255, 255);
9.  
10. imagefilledrectangle($gfx, 0, 0, $w-1, $h-1, $white);
11.  
12. $str='';
13. foreach(range(0, rand(5,7)) as $r)
14. {
15. $str.=chr(rand(65,90));
16. }
17.  
18. $pos=$w/strlen($str);
19.  
20. foreach(range(0,strlen($str)-1) as $s)
21. {
22. $shade=rand(0,100);
23. $tmpgray =  imagecolorallocate($gfx,$shade,$shade,$shade);
24. imagettftext($gfx,
25. rand($h/3,$h/2),
26. rand(-60,60),
27. $s*$pos+($pos*.4),
28. rand($h*.5,$h*.7),
29. $tmpgray,
30. 'arial',
31. $str{$s});
32. }
33.  
34. foreach(range(-$h,$w,5) as $x)
35. {
36. $shade=rand(50,254);
37. $tmpgray =  imagecolorallocate($gfx,$shade, $shade, $shade);
38. imageline($gfx, $x, 0, $x+$h+rand(0,25),$h-1,  $tmpgray);
39. imageline($gfx, $x, $h-1, $x+$h+rand(0,25), 0,  $tmpgray);
40. }
41. //session_start();
42. //$_SESSION['captcha']=$str;
43. //header('Content-type:image/png');
44. //imagepng($gfx);
45.  
46.