fixed that forum list in admin area was displayed incorrectly beginning with PHP 5.4

The reason: "Call-time pass by reference has been removed" in PHP 5.4. There is no more
reference sign on a function call - only on function definitions.

see: http://php.net/manual/en/migration54.incompatible.php

Author: PrimaryFeather

bb-admin/includes/functions.bb-admin.php

@@ -1257,27 +1257,27 @@ class BB_Walker_ForumAdminlistitems extends BB_Walker {
 	var $tree_type = 'forum';
 	var $db_fields = array ('parent' => 'forum_parent', 'id' => 'forum_id'); //TODO: decouple this
 
-	function start_lvl($output, $depth) {
+	function start_lvl(&$output, $depth) {
 		$indent = str_repeat("\t", $depth) . '    ';
 		$output .= $indent . "<ul id='forum-root-$this->forum_id' class='list-block holder'>\n";
 		return $output;
 	}
 
-	function end_lvl($output, $depth) {
+	function end_lvl(&$output, $depth) {
 		$indent = str_repeat("\t", $depth) . '    ';
 		$output .= $indent . "</ul>\n";
 		return $output;
 	}
 
-	function start_el($output, $forum, $depth) {
+	function start_el(&$output, $forum, $depth) {
 		$this->forum_id = $forum->forum_id;
 		$indent = str_repeat("\t", $depth + 1);
 		$output .= $indent . "<li id='forum-$this->forum_id'" . get_alt_class( 'forum', 'forum clear list-block' ) . ">\n";
 
 		return $output;
 	}
 
-	function end_el($output, $forum, $depth) {
+	function end_el(&$output, $forum, $depth) {
 		$indent = str_repeat("\t", $depth + 1);
 		$output .= $indent . "</li>\n";
 		return $output;

bb-includes/class.bb-walker.php

@@ -5,10 +5,10 @@ class BB_Walker {
 	var $db_fields;
 
 	//abstract callbacks
-	function start_lvl($output) { return $output; }
-	function end_lvl($output)   { return $output; }
-	function start_el($output)  { return $output; }
-	function end_el($output)    { return $output; }
+	function start_lvl(&$output) { return $output; }
+	function end_lvl(&$output)   { return $output; }
+	function start_el(&$output)  { return $output; }
+	function end_el(&$output)    { return $output; }
 
 	function _init() {
 		$this->parents = array();
@@ -127,15 +127,15 @@ class BB_Walker_Blank extends BB_Walker { // Used for template functions
 	var $end_lvl   = '';
 
 	//abstract callbacks
-	function start_lvl( $output, $depth ) { 
+	function start_lvl( &$output, $depth ) { 
 		if ( !$this->start_lvl )
 			return '';
 		$indent = str_repeat("\t", $depth);
 		$output .= $indent . "$this->start_lvl\n";
 		return $output;
 	}
 
-	function end_lvl( $output, $depth )   {
+	function end_lvl( &$output, $depth )   {
 		if ( !$this->end_lvl )
 			return '';
 		$indent = str_repeat("\t", $depth);