Description:
------------
First parameter of the libxml_set_external_entity_loader() callback is undefined if argument passed to it is null. 

Test script:
---------------
<?php
ini_set('error_reporting',PHP_INT_MAX-1);
$xml=<<<EOF
<?xml version="1.0"?>
<test/>
EOF;

$xsd=<<<EOF
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:include schemaLocation="nonexistent.xsd"/>
  <xs:element name="test"/>
</xs:schema>
EOF;

libxml_set_external_entity_loader(function($p,$s,$c) {
    var_dump($p,$s,$c);
    die();
});

$dom=new DOMDocument($xml);
var_dump($dom->schemaValidateSource($xsd));

Expected result:
----------------
NULL
string(15) "nonexistent.xsd"
array(4) {
  ["directory"]=>
  NULL
  ["intSubName"]=>
  NULL
  ["extSubURI"]=>
  NULL
  ["extSubSystem"]=>
  NULL
}

Actual result:
--------------
Notice: Undefined variable: p in /home/huge/test.php on line 17
NULL
string(15) "nonexistent.xsd"
array(4) {
  ["directory"]=>
  NULL
  ["intSubName"]=>
  NULL
  ["extSubURI"]=>
  NULL
  ["extSubSystem"]=>
  NULL
}