wp_generate_tag_cloud

function wp_generate_tag_cloud( $tags, $args = '' ) {
	global $wp_rewrite;
	$defaults = array(
		'smallest' => 8, 'largest' => 22, 'unit' => 'pt', 'number' => 0,
		'format' => 'flat', 'separator' => "\n", 'orderby' => 'name', 'order' => 'ASC',
		'topic_count_text_callback' => 'default_topic_count_text',
		'topic_count_scale_callback' => 'default_topic_count_scale', 'filter' => 1,
	);

	if ( !isset( $args['topic_count_text_callback'] ) && isset( $args['single_text'] ) && isset( $args['multiple_text'] ) ) {
		$body = 'return sprintf (
			_n(' . var_export($args['single_text'], true) . ', ' . var_export($args['multiple_text'], true) . ', $count),
			number_format_i18n( $count ));';
		$args['topic_count_text_callback'] = create_function('$count', $body);
	}

	$args = wp_parse_args( $args, $defaults );
	extract( $args );

	if ( empty( $tags ) )
		return;

	$tags_sorted = apply_filters( 'tag_cloud_sort', $tags, $args );
	if ( $tags_sorted != $tags  ) { // the tags have been sorted by a plugin
		$tags = $tags_sorted;
		unset($tags_sorted);
	} else {
		if ( 'RAND' == $order ) {
			shuffle($tags);
		} else {
			// SQL cannot save you; this is a second (potentially different) sort on a subset of data.
			if ( 'name' == $orderby )
				uasort( $tags, '_wp_object_name_sort_cb' );
			else
				uasort( $tags, '_wp_object_count_sort_cb' );

			if ( 'DESC' == $order )
				$tags = array_reverse( $tags, true );
		}
	}

	if ( $number > 0 )
		$tags = array_slice($tags, 0, $number);

	$counts = array();
	$real_counts = array(); // For the alt tag
	foreach ( (array) $tags as $key => $tag ) {
		$real_counts[ $key ] = $tag->count;
		$counts[ $key ] = $topic_count_scale_callback($tag->count);
	}

	$min_count = min( $counts );
	$spread = max( $counts ) - $min_count;
	if ( $spread <= 0 )
		$spread = 1;
	$font_spread = $largest - $smallest;
	if ( $font_spread < 0 )
		$font_spread = 1;
	$font_step = $font_spread / $spread;

	$a = array();

	foreach ( $tags as $key => $tag ) {
		$count = $counts[ $key ];
		$real_count = $real_counts[ $key ];
		$tag_link = '#' != $tag->link ? esc_url( $tag->link ) : '#';
		$tag_id = isset($tags[ $key ]->id) ? $tags[ $key ]->id : $key;
		$tag_name = $tags[ $key ]->name;
		$a[] = "<a href='$tag_link' class='tag-link-$tag_id' title='" . esc_attr( call_user_func( $topic_count_text_callback, $real_count ) ) . "' style='font-size: " .
			( $smallest + ( ( $count - $min_count ) * $font_step ) )
			. "$unit;'>$tag_name</a>";
	}

	switch ( $format ) :
	case 'array' :
		$return =& $a;
		break;
	case 'list' :
		$return = "<ul class='wp-tag-cloud'>\n\t<li>";
		$return .= join( "</li>\n\t<li>", $a );
		$return .= "</li>\n</ul>\n";
		break;
	default :
		$return = join( $separator, $a );
		break;
	endswitch;

	if ( $filter )
		return apply_filters( 'wp_generate_tag_cloud', $return, $tags, $args );
	else
		return $return;
}