Check arrays for recursion

The following function is simpler[opinion] than the code in the accepted answer, and seems to work for any use case that I have been able to contrive. It also seems to be surprisingly fast, typically taking microseconds, though I have not done extensive benchmarking. If there is a problem with it, I would be grateful if somebody could point it out?

// returns TRUE iff the passed object or array contains
// a self-referencing object or array
function is_r($obj, &$visited=array())
  {
  $visited[] = $obj;
  foreach ($obj as $el)
    {
    if (is_object($el) || is_array($el))
      {
      if (in_array($el, $visited, TRUE))
        return TRUE;
      if (is_r($el, $visited))
        return TRUE;
      }
    }
  return FALSE;
  }

It's always fun to try solving "impossible" problems!

Here's a function that will detect recursive arrays if the recursion happens at the top level:

function is_recursive(array &$array) {
    static $uniqueObject;
    if (!$uniqueObject) {
        $uniqueObject = new stdClass;
    }

    foreach ($array as &$item) {
        if (!is_array($item)) {
            continue;
        }

        $item[] = $uniqueObject;
        $isRecursive = end($array) === $uniqueObject;
        array_pop($item);
        if ($isRecursive) {
            return true;
        }
    }

    return false;
}

See it in action.

Detecting recursion at any level would obviously be more tricky, but I think we can agree that it seems doable.

Update

And here is the recursive (pun not intended but enjoyable nonetheless) solution that detects recursion at any level:

function is_recursive(array &$array, array &$alreadySeen = array()) {
    static $uniqueObject;
    if (!$uniqueObject) {
        $uniqueObject = new stdClass;
    }

    $alreadySeen[] = &$array;

    foreach ($array as &$item) {
        if (!is_array($item)) {
            continue;
        }

        $item[] = $uniqueObject;
        $recursionDetected = false;
        foreach ($alreadySeen as $candidate) {
            if (end($candidate) === $uniqueObject) {
                $recursionDetected = true;
                break;
            }
        }

        array_pop($item);

        if ($recursionDetected || is_recursive($item, $alreadySeen)) {
            return true;
        }
    }

    return false;
}

See it in action.

Of course this can also be written to work with iteration instead of recursion by keeping a stack manually, which would help in cases where a very large recursion level is a problem.