Error 500- Internal Server Error bei einer Registerkarte der Seitennavigation

* arrange pages into two parts: top level pages and children_pages
     * children_pages is two dimensional array, eg. 
     * children_pages[10][] contains all sub-pages whose parent is 10. 
     * It only takes O(N) to arrange this and it takes O(1) for subsequent lookup operations
     * If searching, ignore hierarchy and treat everything as top level
     */
    if ( empty($_GET['s']) )  {
        
        $top_level_pages = array();
        $children_pages  = array();
        
        foreach ( $pages as $page ) {
            
            // catch and repair bad pages
            if ( $page->post_parent == $page->ID ) {
                $page->post_parent = 0;
                $wpdb->query( $wpdb->prepare("UPDATE $wpdb->posts SET post_parent = '0' WHERE ID = %d", $page->ID) );
                clean_page_cache( $page->ID );
            }
    
            if ( 0 == $page->post_parent )
                $top_level_pages[] = $page;
            else
                $children_pages[ $page->post_parent ][] = $page;
        }


        $pages = &$top_level_pages;
    }


    $count = 0;
    $start = ($pagenum - 1) * $per_page;
    $end = $start + $per_page;
    
    foreach ( $pages as $page ) {
        if ( $count >= $end )
            break;


        if ( $count >= $start )
            echo "\t" . display_page_row( $page, $level );


        $count++;


        if ( isset($children_pages) )
            _page_rows( $children_pages, $count, $page->ID, $level + 1, $pagenum, $per_page );
    }
    
    // if it is the last pagenum and there are orphaned pages, display them with paging as well
    if ( isset($children_pages) && $count < $end ){
        foreach( $children_pages as $orphans ){
            foreach ( $orphans as $op ) {
                if ( $count >= $end )
                    break;
                if ( $count >= $start )
                    echo "\t" . display_page_row( $op, 0 );
                $count++;
            }
        }
    }
}