Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc.

If possible, make the script source available online and provide
an URL to it here. Try to avoid embedding huge scripts into the report.

In order to get the small code, I began deleting and commenting out parts of the scripts watching for the notice to remain. But at some point I found that deleting, commenting out a line or any other changes made the notice disappear. That is the buggy behaviour, I have no idea why this is so. In order to make the code simple, I made larger parts of it not executable with if (0) etc, so that you do not have to understand them even though they are there. The parts are marked as "not working". The point is if I physically remove them and the script is smaller (or even if I double a line) the notice is not displayed, so maybe the size matters. I provide the final code that does display the notice though it should not, and the script is also available at http://hotelsys.biz/dev_ref.php (the result) and at http://hotelsys.biz/dev_ref.php?src=1 (the source code). 

The notice is about return $oNewNode; in function &add(). In the function, I create $oNewNode=false; to prevent the notice, the 2nd branch is executed, and then the return goes. Before calling add(), render() is called. It does almost nothing to the object but is necessary for the notice to appear.

The code follows

<?

if (isset($_GET["src"])){
	highlight_file("dev_ref.php");
	exit;
	}

error_reporting(E_ALL);

$TextNode=new Node();
$TextNode->render();
$TextNode->add('H', 123);

//------------------------------------------------------------------------

class Node{
	var $hashValues;		//any values
	var $aNodeGroups;		//
	var $sTemplate;			//
	var $bDisplay=true;	//don't hide
	var $bCleanup=true;	//autodelete subnodes when used
	//------------------------------------------------------------------------
	
	function Node($hashValues=false, $bCleanup=true){
	$this->hashValues=array();
	}

	function &add($mixKey, $mixVal=array()){
	$oNewNode=false;
	if (is_array($mixKey)){
		if (0){
			//not working!!!
			foreach($mixKey as $n => $v) $this->add($n, $v);
			}
		}
	else if (!is_array($mixVal) || key($mixVal)===0){
		$this->hashValues[$mixKey]=$mixVal;
		}
	else{
		if (0){
			//not working!!!
			$oNewNode=&new Node($mixVal, $this->bCleanup);
			if (!@$this->aNodeGroups[$mixKey]){
				$this->aNodeGroups[$mixKey]=array();
				}
			$this->aNodeGroups[$mixKey][]=&$oNewNode;
			//--not working!!!
			}
		}
	return $oNewNode;
	}

	function render(){
	
 	if (!$this->bDisplay) return "";
	if ($this->aNodeGroups){
		if (0){
			//not working!!!
			foreach($this->aNodeGroups as $GrpName => $aNodes){
				$bDebug=($GrpName=='Content');
				$this->_EndPos=0;	$sAccumulator=false;
				while(is_string($sTemplate=$this->getGrpTemplate($GrpName))){
					if ($sAccumulator===false){
						$sAccumulator=''; $NodeNum=count($aNodes);
						for($i=0; $i<$NodeNum; $i++){
							if ($sTemplate) $aNodes[$i]->sTemplate=$sTemplate;
							$sAccumulator.=$aNodes[$i]->render();
							if ($this->bCleanup) $aNodes[$i]=false;
							else $aNodes[$i]->sTemplate='';
							}
						}
					$this->putLastGrpText($sAccumulator);
					}
			 	}
			
			if ($this->bCleanup) $this->aNodeGroups=false;
			//--not working!!!
			}
		}

	$Templ=$this->sTemplate;
	
	foreach($this->hashValues as $sKey => $mixVal){
		continue;
		//not working!!!
		$sToken='{{'.$sKey.'}}';
		if (!is_array($mixVal)){
			$Templ=str_replace($sToken, $mixVal, $Templ);
			}
		else{
			$Len=count($mixVal); $sAccumulator='';
			for($ValNo=0; $ValNo<$Len; $ValNo++){
				$sAccumulator.=str_replace($sToken, $mixVal[$ValNo], $Templ);
				}
			$Templ=$sAccumulator;
			}
		//---not working!!!
		}

	if ($this->bCleanup) $this->hashValues=array();

	return $Templ;
	}
}

?>

Please try using this CVS snapshot:

  http://snaps.php.net/php4-STABLE-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php4-win32-STABLE-latest.zip

Can't reproduce. This code doesn't produce any warning/notices for me neither with 5.x, nor with 4.4.

Afraid I won't be able to test with the latest release

We cannot fix something that we can't replicate.
Feel free to repoen the report when you're able to provide a short reproduce case that doesn't work as expected with the latest CVS.