Fix #52751: XPath processing-instruction() function is not supported.

ext/simplexml/simplexml.c

@@ -1313,7 +1313,7 @@ PHP_METHOD(SimpleXMLElement, xpath)
 
 		for (i = 0; i < result->nodeNr; ++i) {
 			nodeptr = result->nodeTab[i];
-			if (nodeptr->type == XML_TEXT_NODE || nodeptr->type == XML_ELEMENT_NODE || nodeptr->type == XML_ATTRIBUTE_NODE) {
+			if (nodeptr->type == XML_TEXT_NODE || nodeptr->type == XML_ELEMENT_NODE || nodeptr->type == XML_ATTRIBUTE_NODE || nodeptr->type == XML_PI_NODE) {
 				/**
 				 * Detect the case where the last selector is text(), simplexml
 				 * always accesses the text() child by default, therefore we assign

ext/simplexml/tests/req52751.phpt

@@ -0,0 +1,58 @@
+--TEST--
+Request #52751 (XPath processing-instruction() function is not supported)
+--EXTENSIONS--
+simplexml
+--FILE--
+<?php
+
+$xml = <<<XML
+<?xml version="1.0" encoding="utf-8"?>
+<foo>
+  <bar>text node</bar>
+  <bar><?baz href="ftw" ?></bar>
+  <bar><?foo bar ?></bar>
+</foo>
+XML;
+
+$sxe = simplexml_load_string($xml);
+
+var_dump(
+    $sxe->xpath('//bar')
+);
+
+var_dump(
+    $sxe->xpath('//processing-instruction(\'baz\')')
+);
+
+foreach ($sxe->xpath('//processing-instruction()') as $pi) {
+    var_dump($pi->getName());
+}
+
+?>
+--EXPECT--
+array(3) {
+  [0]=>
+  object(SimpleXMLElement)#2 (1) {
+    [0]=>
+    string(9) "text node"
+  }
+  [1]=>
+  object(SimpleXMLElement)#3 (1) {
+    ["baz"]=>
+    object(SimpleXMLElement)#5 (0) {
+    }
+  }
+  [2]=>
+  object(SimpleXMLElement)#4 (1) {
+    ["foo"]=>
+    object(SimpleXMLElement)#5 (0) {
+    }
+  }
+}
+array(1) {
+  [0]=>
+  object(SimpleXMLElement)#4 (0) {
+  }
+}
+string(3) "baz"
+string(3) "foo"