Here's a more detailed example
<?php
// Sig Image
header("Content-type: image/gif");
// Make Canvas
$img = ImageCreate(400, 50);
$black = ImageColorAllocate($img, 0, 0, 0);
$white = ImageColorAllocate($img, 255, 255, 255);
// Fill the canvas with white
ImageFilledRectangle ($img, 0, 0, 400, 50, $white);
// Random Phrases
$text = array(
"This is a random phrase",
"This is also a random phrase",
"I think you get the idea.",
"Add as many of these as you like.");
// Look at this, DavidT
srand(time());
$num = (rand() % 4);
// Stick the Text in There
ImageString($img, 10, 10, 5, $text[ $num ], $black);
// Complete
ImageGIF($img);
?>