Create a flat list of id from tree (respecting tree display order)

tree_to_flat_id_list.php

<?php 

/**
 * Create a flat list of id (respecting tree display order).
 * Assumes that $table is keeping tree structure using "id", "parent_id", "display_order" fields
 * @param string $table name of the table keeping tree data
 * @param integer $start_node_id the id of the start node
 * @param boolean $include_start_node include (or not) the id of the start node in returning result
 * @param string $f_id name of "id" field
 * @param string $f_parent_id name of "parent_id" field
 * @param string $f_order name of "display_order" field
 * @return array
 *
 * LICENSE: GNU General Public License (http://www.gnu.org/copyleft/gpl.html)
 *
 */
function tree_to_flat_id_list($table, $start_node_id, $include_start_node = true,
    $f_id = 'id', $f_parent_id = 'parent_id', $f_order = 'display_order') {

    global $dbi; // simple custom wrapper of php-ADODB
    static $array_ids = array();

    if ($include_start_node) {
        $sql = 'SELECT * FROM ' . $table . " WHERE " . $f_id . "=" . $start_node_id;
        $res = $dbi->select($sql);
        if ($res === false)
            trigger_error('Wrong SQL: ' . $dbi->lastSQL . ' Error: ' . $dbi->lastError, E_USER_ERROR);
        $array_ids[] = $res[0][$f_id];
    }

    $sql = 'SELECT * FROM ' . $table;
    if ($start_node_id > 0) {
        $sql .= ' WHERE ' . $f_parent_id . '=' . $start_node_id;
    } else {
        $sql .= ' WHERE ' . $f_parent_id . ' IS NULL';
    }
    $sql .= ' ORDER BY ' . $f_order;

    $branch = $dbi->select($sql);
    if ($branch === false)
        trigger_error('Wrong SQL: ' . $dbi->lastSQL . ' Error: ' . $dbi->lastError, E_USER_ERROR);

    if (count($branch) > 0) {
        foreach ($branch as $node) {
            $array_ids[] = $node[$f_id];
            tree_to_flat_id_list($table, $node[$f_id], false);
        }
    }

    return $array_ids;
}

?>