Fix viable next sibling search for replaceWith

Closes GH-11888.

Author: nielsdos

ext/dom/parentnode.c

@@ -550,25 +550,45 @@ void dom_child_node_remove(dom_object *context)
 
 void dom_child_replace_with(dom_object *context, zval *nodes, int nodesc)
 {
+	/* Spec link: https://dom.spec.whatwg.org/#dom-childnode-replacewith */
+
 	xmlNodePtr child = dom_object_get_node(context);
+
+	/* Spec step 1 */
 	xmlNodePtr parentNode = child->parent;
+	/* Spec step 2 */
+	if (!parentNode) {
+		int stricterror = dom_get_strict_error(context->document);
+		php_dom_throw_error(HIERARCHY_REQUEST_ERR, stricterror);
+		return;
+	}
 
 	int stricterror = dom_get_strict_error(context->document);
 	if (UNEXPECTED(dom_child_removal_preconditions(child, stricterror) != SUCCESS)) {
 		return;
 	}
 
-	xmlNodePtr insertion_point = child->next;
+	/* Spec step 3: find first following child not in nodes; otherwise null */
+	xmlNodePtr viable_next_sibling = child->next;
+	while (viable_next_sibling) {
+		if (!dom_is_node_in_list(nodes, nodesc, viable_next_sibling)) {
+			break;
+		}
+		viable_next_sibling = viable_next_sibling->next;
+	}
 
 	if (UNEXPECTED(dom_sanity_check_node_list_for_insertion(context->document, parentNode, nodes, nodesc) != SUCCESS)) {
 		return;
 	}
 
+	/* Spec step 4: convert nodes into fragment */
 	xmlNodePtr fragment = dom_zvals_to_fragment(context->document, parentNode, nodes, nodesc);
 	if (UNEXPECTED(fragment == NULL)) {
 		return;
 	}
 
+	/* Spec step 5: perform the replacement */
+
 	xmlNodePtr newchild = fragment->children;
 	xmlDocPtr doc = parentNode->doc;
 
@@ -580,7 +600,7 @@ void dom_child_replace_with(dom_object *context, zval *nodes, int nodesc)
 	if (newchild) {
 		xmlNodePtr last = fragment->last;
 
-		dom_pre_insert(insertion_point, parentNode, newchild, fragment);
+		dom_pre_insert(viable_next_sibling, parentNode, newchild, fragment);
 
 		dom_fragment_assign_parent_node(parentNode, fragment);
 		dom_reconcile_ns_list(doc, newchild, last);

ext/dom/tests/replaceWith_non_viable_next_sibling.phpt

@@ -0,0 +1,36 @@
+--TEST--
+replaceWith() with a non-viable next sibling
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$doc = new DOMDocument;
+$doc->loadXML(<<<XML
+<?xml version="1.0"?>
+<container>
+    <child>
+        <alone/>
+    </child>
+</container>
+XML);
+
+$container = $doc->documentElement;
+$child = $container->firstElementChild;
+$alone = $child->firstElementChild;
+
+$child->after($alone);
+echo $doc->saveXML();
+$child->replaceWith($alone);
+echo $doc->saveXML();
+?>
+--EXPECT--
+<?xml version="1.0"?>
+<container>
+    <child>
+        
+    </child><alone/>
+</container>
+<?xml version="1.0"?>
+<container>
+    <alone/>
+</container>