need help with captcha

[tinman]

So far I have gotten the background image to generate as randomly as i want but i cant seem to get text to show up. This is my code:

 

$im = imagecreate(400, 30);

$white = imagecolorallocate($im, 255, 0, 0);

$black = imagecolorallocate($im, 0, 0, 0);

imagefilledrectangle($im, 0, 0, 399, 29, $white);

$text = 'Testing...';

$font = 'arial.ttf';

imagettftext($im, 20, 0, 10, 20, $black, $font, $text);

imagepng($im);

imagedestroy($im);

 

but when I try to load it, no text appears. Do I need to incorporate my own font file into it? If so how do i do that? Or is it something even more simple that I have missed? Thank you for your time and assistance.