Under some circumstances PHP seems to lose track of the number of references it has to a variable (at least, that is what I think is happening).

In the following script, look for the line "/** remove the following line and things break **/" in TelesInvoiceParser::_endElement().

The critical lines are:

$this->currentElement =& $this->currentElement->parent;

which fails, and:

$p =& $this->currentElement->parent;
$this->currentElement =& $this->currentElement->parent;

which works. It appears that the dummy reference $p is needed for the parent property to continue to exist out of the scope of the method.

======== Explanation of script ================


What is happening is that as an XML document is parsed, every time a new element is found a matching object is created and set to be the new "currentElement" and this new currentElement's "parent" property is set to a reference to the previous currentElement. In this way when the element is closed we can get back to its enclosing element.
 

============= PHP configure flags ==============
./configure --with-apache=../apache_1.3.19/ --with-mysql --with-xslt-sablot --enable-xslt --with-expat-dir=/usr


The input XML is
======== XML ===========

<invoice>
  <from>
    <date day="15" month="12" year="2001"/>
  </from>

  <to>
    <date day="14" month="12" year="2001"/>
  </to>

  <issuer>1</issuer>

  <person id="1234">
    <order>4567</order>
    <traffic no="0" rate="0" zone="2">24</traffic>
    <traffic no="1" rate="4" zone="2">48</traffic>
    <traffic no="2" rate="6" zone="2">96</traffic>
    <traffic no="3" rate="8" zone="2">37</traffic>
  </person>
</invoice>


========== END XML ==============

========== PHP ===================
<?
////
//  Class to parse a Xc Invoice XML Document
////

class XcXMLElement {
	var $parent = null;
	var $cdata = "";
	
	function setA($attributes) {
		global $indent;
		$indent.="&nbsp; &nbsp; ";
		echo "<p>$indent Setting up a new ".get_class($this)."</p>\n";
		foreach($attributes as $name => $value) {
			$this->$name = $value;
		}
	}

	function setD($cdata) {
		$this->cdata .= $cdata;
	}

	function end() {
		global $indent;
		echo "<p>$indent end of element ".get_class($this)."</p>\n";
		$indent = substr($indent,0,strlen($indent) - 14);
	}
}

class XcInvoice extends XcXMLElement {
	var $from, $to, $issuer, $person=array();
	function end() {
		$this->parent->invoice =& $this;
		parent::end();
	}
}

class XcFrom extends XcXMLElement  {
	var $date;
	function end() {
		$this->parent->from =& $this;
		parent::end();
	}
}

class XcTo extends XcXMLElement  {
	var $date;
	function end() {
		$this->parent->to =& $this;
		parent::end();
	}
}

class XcDate extends XcXMLElement  {
	var $day, $month, $year;

	function timestamp() {
		return $this->year.$this->leading($this->month,2).$this->leading($this->day,2)."000000";
	}

	function leading($value,$length) {
	////
	//  Make sure there are the right number of leading zeros on a number
	////
		$num = "0000$value";
		return substr($value,-$length);
	}

	function end() {
		$this->parent->date =& $this;
		parent::end();
	}
}

class XcPerson extends XcXMLElement  {
	var $id;
	var $order;
	var $traffic = array();

	function end() {
	////
	//  When a complete person object has been read, we write its data to the database
	////
		global $indent;
		echo "$indent Writing a person to the database<br>";
		echo "$indent Data for the period ".$this->parent->from->date->timestamp()." to ".$this->parent->to->date->timestamp()."<br>\n";
		parent::end();
	}

}

class XcOrder extends XcXMLElement { }

class XcIssuer extends XcXMLElement { }

class XcTraffic extends XcXMLElement  {
	var $no, $rate, $zone;
	var $class="XcTraffic";
}


$indent="";

class XcInvoiceParser {
	var $fp;
	var $invoice; // Invoice currently being constructed
	var $fromDate;
	var $toDate;
	var $xmlParser;

	function XcInvoiceParser($filename) {

		$this->elementHandlers = array(
			"invoice" 	=> new XcInvoice,
			"from" 		=> new XcFrom,
			"to" 		=> new XcTo,
			"date" 		=> new XcDate,
			"issuer" 	=> new XcIssuer,
			"person" 	=> new XcPerson,
			"order" 	=> new XcOrder,
			"traffic" 	=> new XcTraffic
		);


		$this->fp = fopen($filename,"r");
		$this->state = null;
		$this->invoice = null;
		$this->fromDate = null;
		$this->toDate = null;
		$this->issuer = null;
		$this->xmlParser = xml_parser_create();
		xml_set_object($this->xmlParser, &$this);
		xml_set_element_handler($this->xmlParser,"startElement","endElement");
		xml_set_character_data_handler($this->xmlParser, "cdata");
		xml_parser_set_option($this->xmlParser,XML_OPTION_CASE_FOLDING,false);
		$this->level = 0;
	}

	function startElement($parser,$element,$attributes) {
		$this->level++;
		echo "start element $element at level $this->level <br>\n"; flush();
		if($handler = $this->elementHandlers[$element]) {
			$handler->parent =& $this->currentElement;
			$this->currentElement =& $handler;
			$this->currentElement->setA($attributes);
		} else {
			echo "Unknown element $element";
			exit;
		}
	}

	function endElement($parser,$element) {
		$this->_endElement($parser, $element);
		echo "Ended OK"; flush();
	}
	function _endElement($parser,$element) {

		echo "ending element $element at level $this->level .."; flush();
		$this->currentElement->end();
		$this->level--;
		if(!$this->currentElement->parent) {
			echo "Oops!! ";
			exit;
		}
		echo "popping up the stack from ".get_class($this->currentElement)." to ".get_class($this->currentElement->parent)."..<br>\n";
		flush();

/** remove the following line and things break!! **/
		$p =& $this->currentElement->parent;

		$this->currentElement =& $this->currentElement->parent;
		echo "popped<br>\n"; flush();
		echo "currentElement is now ".get_class($this->currentElement)."<br>\n";
		flush();
	}

	function cdata($parser,$cdata) {
		if(!$this->currentElement) {
			echo "Can't set CDATA on a null element<br>\n";
			flush();
			exit;
		}
		$this->currentElement->setD($cdata);
	}

	function parse() {
		while($line = fread($this->fp,4096)) {
			if(!xml_parse($this->xmlParser, $line, feof($this->fp))) {
				echo "ERROR READING FILE";
				exit;
			}
		}
	}
}

$p = new XcInvoiceParser("/tmp/toto");
$p->parse();
?>

===================== END PHP ========================