Fix DOMElement->prefix with empty string creates bogus prefix

Closes GH-12770.

Author: nielsdos

ext/dom/node.c

@@ -674,8 +674,14 @@ zend_result dom_node_prefix_write(dom_object *obj, zval *newval)
 			prefix_str = Z_STR_P(newval);
 
 			prefix = ZSTR_VAL(prefix_str);
+			if (*prefix == '\0') {
+				/* The empty string namespace prefix does not exist.
+				 * We should fall back to the default namespace in this case. */
+				prefix = NULL;
+			}
 			if (nsnode && nodep->ns != NULL && !xmlStrEqual(nodep->ns->prefix, (xmlChar *)prefix)) {
 				strURI = (char *) nodep->ns->href;
+				/* Validate namespace naming constraints */
 				if (strURI == NULL ||
 					(zend_string_equals_literal(prefix_str, "xml") && strcmp(strURI, (char *) XML_XML_NAMESPACE)) ||
 					(nodep->type == XML_ATTRIBUTE_NODE && zend_string_equals_literal(prefix_str, "xmlns") &&

ext/dom/tests/DOMElement_prefix_empty.phpt

@@ -0,0 +1,51 @@
+--TEST--
+DOMElement->prefix with empty string creates bogus prefix
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+
+$dom = new DOMDocument;
+$dom->loadXML("<hello:container xmlns:conflict=\"urn:foo\" xmlns:hello=\"http://www.w3.org/1999/xhtml\"/>");
+
+$container = $dom->documentElement;
+
+echo "--- Changing the prefix to an empty string ---\n";
+
+$container->prefix = "";
+echo $dom->saveXML();
+
+echo "--- Changing the prefix to an empty C-style string ---\n";
+
+$container->prefix = "\0foobar";
+echo $dom->saveXML();
+
+echo "--- Changing the prefix to \"hello\" ---\n";
+
+$container->prefix = "hello";
+echo $dom->saveXML();
+
+echo "--- Changing the prefix to that of a conflicting namespace (\"conflict\") ---\n";
+
+try {
+    $container->prefix = "conflict";
+} catch (DOMException $e) {
+    echo $e->getMessage(), "\n";
+}
+echo $dom->saveXML();
+
+?>
+--EXPECT--
+--- Changing the prefix to an empty string ---
+<?xml version="1.0"?>
+<container xmlns:conflict="urn:foo" xmlns:hello="http://www.w3.org/1999/xhtml" xmlns="http://www.w3.org/1999/xhtml"/>
+--- Changing the prefix to an empty C-style string ---
+<?xml version="1.0"?>
+<container xmlns:conflict="urn:foo" xmlns:hello="http://www.w3.org/1999/xhtml" xmlns="http://www.w3.org/1999/xhtml"/>
+--- Changing the prefix to "hello" ---
+<?xml version="1.0"?>
+<hello:container xmlns:conflict="urn:foo" xmlns:hello="http://www.w3.org/1999/xhtml" xmlns="http://www.w3.org/1999/xhtml"/>
+--- Changing the prefix to that of a conflicting namespace ("conflict") ---
+Namespace Error
+<?xml version="1.0"?>
+<hello:container xmlns:conflict="urn:foo" xmlns:hello="http://www.w3.org/1999/xhtml" xmlns="http://www.w3.org/1999/xhtml"/>