PHP Fatal error: Cannot pass parameter N by reference in错误原因及解决办法

<?php
function math_add($p1,&$p2){
    return $p1+$p2;
}
$p = 2;
echo math_add(1,$p);//正确：输出3
echo math_add(1,2);//错误：PHP Fatal error:  Cannot pass parameter 2 by reference
?>

foreach ($this->metas as $key => $val ) {
	${"meta" . $i} = $this->xml->createElement($key);
	${"meta" . $i}->appendChild($this->xml->createTextNode($val));
	$articleinfo->appendChild(${"meta" . $i}); //应输入变量而不是"meta" . $i
	$i++;
}

public function appendChild(&$child)
{
	$child->parentNode = &$this;
	$this->childNodes[] = $child;
 
	if ($child->nodeType == XML_TYPE_NODE) {
		$child->previousSibling = &$this->lastChild;
	}
 
	if ($this->nodeType == XML_TYPE_NODE) {
		if (!is_null($this->lastChild)) {
			$this->lastChild->nextSibling = $child;
		}
 
		$this->firstChild = &$this->childNodes[0];
		$this->lastChild = $child;
	}
}