function getCloud( $data = array(), $minFontSize = 12, $maxFontSize = 30 ) {
	$minimumCount = min($data);
	$maximumCount = max($data);
	$spread = $maximumCount - $minimumCount;
	$cloudHTML = '';
	$cloudTags = array();
	
	$spread == 0 && $spread = 1;
	
	foreach( $data as $tag => $count ) {
		$size = $minFontSize + ( $count - $minimumCount )* ( $maxFontSize - $minFontSize ) / $spread;
		$cloudTags[] = '<a style="font-size: ' . floor( $size ) . 'px'. '" class="tag_cloud" href="#" title="\'' . $tag .'\' returned a count of ' . $count . '">'.htmlspecialchars(stripslashes( $tag ) ) . '</a>';
	}
    return join( "\n", $cloudTags ) . "\n";
}

Esta función y algunas más, todas ellas muy interesantes en Handy PHP Code Snippets.