<?php
$imageArray = range(1,100);
$c = count($imageArray);
$tab = "\t";
$itemsPerBlock = 8;

echo '<ul class="thumbs">'."\n";

for ($i = 0; $i < $c; $i++){ // Storing the count in a variable ($c) will increase your script speed/resources.
	if($i > 0 && ($i % $itemsPerBlock) === 0){
		// Modulus (%) will simply return the remainder of deviding the numbers. 6 % 4 = 2;  27 % 8 = 3;
		echo '</ul>' . PHP_EOL . '<ul class="thumbs">' . PHP_EOL;
		// PHP_EOL is a php constant for End Of Line. Depending on the type of server. (Unix, Windows, Mac), it will use the appropriate signal.
	}
	echo "\t" . '<li><a href="images/gallery/model/' . $imageArray[$i] . '" /><img src="images/gallery/thumbs/' . $imageArray[$i] . '" /></a></li>' . PHP_EOL;
}
echo '</ul>';
?>