check circular referencing in flat array in PHP

checkCircular.php

private static function isHierarchyCircular ($hierarchy, $circularIdentifier, $nextIdentifier) {
	if (!array_key_exists($nextIdentifier, $hierarchy)) {
		$hierarchy[$nextIdentifier] = array();
	}
	
	$startHierarchy = $hierarchy[$nextIdentifier];
	
	if (in_array($circularIdentifier, $startHierarchy)) {
		return true;
	}
	else {
		foreach ($startHierarchy as $subStartHierarchy) {
			if (self::isHierarchyCircular($hierarchy, $circularIdentifier, $subStartHierarchy)) {
				return true;
			}
		}
	}
	
	return false;
}