Fixed bug #76777 and added test

Set undefined values to null rather than undefined.

NEWS

@@ -17,6 +17,10 @@ PHP                                                                        NEWS
   . Fixed bug #74484 (MessageFormatter::formatMessage memory corruption with
     11+ named placeholders). (Anatol)
 
+- libxml:
+  . Fixed bug #76777 ("public id" parameter of libxml_set_external_entity_loader
+    callback undefined). (Ville Hukkamäki)
+
 - mbstring:
   . Fixed bug #76704 (mb_detect_order return value varies based on argument
     type). (cmb)

ext/libxml/libxml.c

@@ -591,12 +591,12 @@ static xmlParserInputPtr _php_libxml_external_entity_loader(const char *URL,
 	if (ID != NULL) {
 		ZVAL_STRING(&params[0], ID);
 	} else {
-		ZVAL_UNDEF(&params[0]);
+		ZVAL_NULL(&params[0]);
 	}
 	if (URL != NULL) {
 		ZVAL_STRING(&params[1], URL);
 	} else {
-		ZVAL_UNDEF(&params[1]);
+		ZVAL_NULL(&params[1]);
 	}
 	ctxzv = &params[2];
 	array_init_size(ctxzv, 4);

ext/libxml/tests/bug76777.phpt

@@ -0,0 +1,43 @@
+--TEST--
+Bug #76777 (first parameter of libxml_set_external_entity_loader callback undefined)
+--SKIPIF--
+<?php
+if (!extension_loaded('libxml')) die('skip');
+if (getenv("SKIP_ONLINE_TESTS")) die('skip online test');
+?>
+--FILE--
+<?php
+$xml=<<<EOF
+<?xml version="1.0"?>
+<test/>
+EOF;
+
+$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);
+$dom->schemaValidateSource($xsd);
+?>
+--EXPECTF--
+NULL
+string(15) "nonexistent.xsd"
+array(4) {
+  ["directory"]=>
+  NULL
+  ["intSubName"]=>
+  NULL
+  ["extSubURI"]=>
+  NULL
+  ["extSubSystem"]=>
+  NULL
+}