A simple example:
To make one line of text fit in the image.
<?php
header ("Content-type: image/png");
$string = "[email protected]";
$font = 4;
$width = ImageFontWidth($font) * strlen($string);
$height = ImageFontHeight($font);
$im = @imagecreate ($width,$height);
$background_color = imagecolorallocate ($im, 255, 255, 255); //white background
$text_color = imagecolorallocate ($im, 0, 0,0);//black text
imagestring ($im, $font, 0, 0, $string, $text_color);
imagepng ($im);
?>
I use something like this for spamprotection of my visitors (pass userid as an url-parameter for this php)