Multi-level categories

Combining functions method:

function dropdown_cats() {
	global $table_prefix;
	$main_cats = "SELECT cat_id, cat_name FROM ${table_prefix}categories WHERE cat_parent = 0";
	$cats = mysql_query($main_cats);

	while ( $main_cat_list = mysql_fetch_array($cats) ) {
		$main_cat = stripslashes($main_cat_list['cat_name']);
		echo '<option value="' . $main_cat_list['cat_id'] . "\\">$main_cat</option>\\n";
		dropdown_sub_cats($main_cat_list['cat_id']);
	}
}

function dropdown_sub_cats($main_cat_id, $level = 1) {
	global $table_prefix;

	$query = "SELECT cat_id, cat_name FROM ${table_prefix}categories WHERE cat_parent = $main_cat_id";
	$sub_list = mysql_query($query);
	#$num_rows = mysql_num_rows($sub_list); // Required when display in <ul></ul>

	#if ($num_rows > 0) {
		while ( $sub_cat_list = mysql_fetch_array($sub_list) ) {
			$indent = str_repeat('–', $level);
			$cat_name = stripslashes($sub_cat_list['cat_name']);
			echo '<option value="' . $sub_cat_list['cat_id'] . "\\">$indent$cat_name</option>\\n";
			dropdown_sub_cats($sub_cat_list['cat_id'], $level + 1);
		}
	#}
}

Single function method:

function list_categories($parent = 0, $level = 0) {
	global $l, $table_prefix;
	$results = mysql_query( $l['db_error_query_select'], "SELECT cat_id, cat_name, cat_parent
		FROM {$table_prefix}categories
		WHERE cat_parent = $parent " );

	$tab = str_repeat("\\t", $level*2);
	$tab2 = ( $level > 1 ? str_repeat("\\t", $level*2-1) : "\\t");
	if (mysql_num_rows($results) > 0) {
		echo "\\n$tab<ul>";
		while ( $r = mysql_fetch_array($results) ) {
			if ( $parent == $r['cat_parent'] ) {
				echo "\\n$tab\\t<li><a href=\\"./categories.php?action=view&amp;idn={$r['cat_id']}\\">{$r['cat_name']}</a>";
			}
			list_categories($r['cat_id'], $level+1);
			echo "</li>";
		}
		echo "\\n$tab</ul>\\n$tab2";
	}
	mysql_free_result($results);
}