check your code.
If this is a cut and paste you have typo here:
$length = $isnt->length;
should read
$length = $inst->length;

Sorry for the mockup code. I thought it would be sufficient to describe the problem.

Here is actually executable code.

<?PHP
	error_reporting(E_ALL);

	class NArray Implements ArrayAccess, Iterator {
		
		private $arraylist;
		private $currentIndex;
		
		function __construct() {
			$this->currentIndex = 0;
			$this->arraylist = array();
		}
		
		function offsetExists($offset){
		   if(isset($this->arraylist[$offset])){
		       return TRUE;
		   } else {
		       return FALSE;
		   }
		}

		function offsetGet($offset){
		   return $this->arraylist[$offset];
		}

		function offsetSet($offset, $value){
		   if($offset){
		       $this->arraylist[$offset] = $value;
		   } else {
		       $this->arraylist[] = $value;
		   }
		}

		function offsetUnset($offset){
			unset($this->arraylist[$offset]); 
		}
		
		function current() {
			return $this->arraylist[$this->currentIndex];
		}
		
		function key() {
			return $this->currentIndex;
		}
		
		function next() {
			return $this->arraylist[$this->currentIndex++];
		}
		
		function rewind() {
			$this->currentIndex = 0;
		}
		
		function valid() {
			if(array_key_exists($this->currentIndex, $this->arraylist)) {
				return true;
			} else {
				return false;
			}
		}
		
		function append($value) {
			$this->arraylist->offsetSet(null, $value);
		}
		function length() {
			return count($this->arraylist);
		}

		function getIterator() {
			return $this;
		}

	}
	
	$nArray = new NArray();
	$nArray[0] = 'a';
	$nArray[1] = 'b';
	$nArray[2] = 'c';
	$nArray[3] = 'd';
	
	foreach($nArray as $index => $item) {
		echo $index . ' = '. $item . '<br />';
	}
	
	echo 'Counted Length Outside Class: '. count($nArray) ."<br>";
	echo 'Counted Length Inside Class: '.$nArray->length();
	echo "<br><br><br>";
	show_source(__FILE__);
?>

Please ignore the fact that this should be an arrayobject because it is simply a proof of concept, the use of an arrayaccess interface will be used later to work with a sql dataset without actually loading all the data in that dataset. A count value will be returned from num_rows type commands without an internal array of the data ever existing.

Hope this helps.

Probably PHP 5.1 comes with the following interface which works with count():

Interface Countable
{
    function count()
}

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

Countable is added to 5.1-dev