diff --git a/ext/dom/document.c b/ext/dom/document.c
index 371c9f846273..e4d285c990fe 100644
--- a/ext/dom/document.c
+++ b/ext/dom/document.c
@@ -1757,6 +1757,35 @@ static int dom_perform_xinclude(xmlDocPtr docp, dom_object *intern, zend_long fl
 	return err;
 }
 
+/* For modern DOM, namespace declarations are stored as attributes (node->nsDef
+ * is NULL), so libxml's native validators can't resolve prefixed QNames found in
+ * content (e.g. an xs:QName attribute value). Temporarily relink them, mirroring
+ * what C14N does in dom_canonicalization(). */
+typedef struct {
+	HashTable links;
+	bool active;
+} dom_validate_ns_guard;
+
+static void dom_validate_ns_guard_begin(dom_validate_ns_guard *guard, xmlDocPtr docp)
+{
+	guard->active = php_dom_follow_spec_node((const xmlNode *) docp);
+	if (guard->active) {
+		zend_hash_init(&guard->links, 0, NULL, NULL, false);
+		xmlNodePtr root_element = xmlDocGetRootElement(docp);
+		if (root_element) {
+			dom_relink_ns_decls(&guard->links, root_element);
+		}
+	}
+}
+
+static void dom_validate_ns_guard_end(dom_validate_ns_guard *guard)
+{
+	if (guard->active) {
+		dom_unlink_ns_decls(&guard->links);
+		zend_hash_destroy(&guard->links);
+	}
+}
+
 /* {{{ Substitutues xincludes in a DomDocument */
 PHP_METHOD(DOMDocument, xinclude)
 {
@@ -1832,8 +1861,11 @@ PHP_METHOD(DOMDocument, validate)
 	cvp->userData = NULL;
 	cvp->error    = (xmlValidityErrorFunc) php_libxml_error_handler;
 	cvp->warning  = (xmlValidityErrorFunc) php_libxml_error_handler;
-
-	if (xmlValidateDocument(cvp, docp)) {
+	dom_validate_ns_guard guard;
+	dom_validate_ns_guard_begin(&guard, docp);
+	int dtd_valid = xmlValidateDocument(cvp, docp);
+	dom_validate_ns_guard_end(&guard);
+	if (dtd_valid) {
 		RETVAL_TRUE;
 	} else {
 		RETVAL_FALSE;
@@ -1930,7 +1962,10 @@ static void dom_document_schema_validate(INTERNAL_FUNCTION_PARAMETERS, int type)
 	PHP_LIBXML_SANITIZE_GLOBALS(validate);
 	xmlSchemaSetValidOptions(vptr, valid_opts);
 	xmlSchemaSetValidErrors(vptr, php_libxml_error_handler, php_libxml_error_handler, vptr);
+	dom_validate_ns_guard guard;
+	dom_validate_ns_guard_begin(&guard, docp);
 	is_valid = xmlSchemaValidateDoc(vptr, docp);
+	dom_validate_ns_guard_end(&guard);
 	xmlSchemaFree(sptr);
 	xmlSchemaFreeValidCtxt(vptr);
 	PHP_LIBXML_RESTORE_GLOBALS(validate);
@@ -2028,7 +2063,10 @@ static void dom_document_relaxNG_validate(INTERNAL_FUNCTION_PARAMETERS, int type
 	}
 
 	xmlRelaxNGSetValidErrors(vptr, php_libxml_error_handler, php_libxml_error_handler, vptr);
+	dom_validate_ns_guard guard;
+	dom_validate_ns_guard_begin(&guard, docp);
 	is_valid = xmlRelaxNGValidateDoc(vptr, docp);
+	dom_validate_ns_guard_end(&guard);
 	xmlRelaxNGFree(sptr);
 	xmlRelaxNGFreeValidCtxt(vptr);
 
diff --git a/ext/dom/namespace_compat.h b/ext/dom/namespace_compat.h
index 23c80acc7fd7..185673f8586e 100644
--- a/ext/dom/namespace_compat.h
+++ b/ext/dom/namespace_compat.h
@@ -69,4 +69,11 @@ PHP_DOM_EXPORT php_dom_in_scope_ns php_dom_get_in_scope_ns(php_dom_libxml_ns_map
 PHP_DOM_EXPORT php_dom_in_scope_ns php_dom_get_in_scope_ns_legacy(const xmlNode *node);
 PHP_DOM_EXPORT void php_dom_in_scope_ns_destroy(php_dom_in_scope_ns *in_scope_ns);
 
+/* Temporarily materialize namespace declarations as nsDef entries on the tree so
+ * that libxml's native validators/canonicalizers can resolve prefixed QNames that
+ * appear in element/attribute *content*. Modern DOM keeps declarations off the
+ * tree (node->nsDef == NULL), which xmlSearchNs() cannot follow. */
+PHP_DOM_EXPORT void dom_relink_ns_decls(HashTable *links, xmlNodePtr root);
+PHP_DOM_EXPORT void dom_unlink_ns_decls(HashTable *links);
+
 #endif
diff --git a/ext/dom/node.c b/ext/dom/node.c
index 505f6ee452c3..a3ceaaa14af8 100644
--- a/ext/dom/node.c
+++ b/ext/dom/node.c
@@ -2201,7 +2201,7 @@ static void dom_relink_ns_decls_element(HashTable *links, xmlNodePtr node)
 	}
 }
 
-static void dom_relink_ns_decls(HashTable *links, xmlNodePtr root)
+void dom_relink_ns_decls(HashTable *links, xmlNodePtr root)
 {
 	dom_relink_ns_decls_element(links, root);
 
@@ -2213,7 +2213,7 @@ static void dom_relink_ns_decls(HashTable *links, xmlNodePtr root)
 	}
 }
 
-static void dom_unlink_ns_decls(HashTable *links)
+void dom_unlink_ns_decls(HashTable *links)
 {
 	ZEND_HASH_MAP_FOREACH_NUM_KEY_VAL(links, zend_ulong h, zval *data) {
 		if (h & 1) {
diff --git a/ext/dom/tests/gh22219.phpt b/ext/dom/tests/gh22219.phpt
new file mode 100644
index 000000000000..637a47039c35
--- /dev/null
+++ b/ext/dom/tests/gh22219.phpt
@@ -0,0 +1,54 @@
+--TEST--
+GH-22219 (Dom\XMLDocument::schemaValidate fails to resolve xs:QName value from an in-scope prefix)
+--EXTENSIONS--
+dom
+--SKIPIF--
+<?php
+if (!method_exists('Dom\XMLDocument', 'schemaValidateSource')) die('skip schema validation not available');
+?>
+--FILE--
+<?php
+
+$xml = <<<XML
+<?xml version="1.0" encoding="UTF-8"?>
+<root xmlns="urn:test" xmlns:ref="urn:other">
+  <item target="ref:something"/>
+</root>
+XML;
+
+// The 'ref' prefix is declared on <root> but only used inside the xs:QName
+// attribute value, never as an element or attribute namespace.
+$xsd = <<<XSD
+<?xml version="1.0" encoding="UTF-8"?>
+<schema xmlns="http://www.w3.org/2001/XMLSchema"
+        targetNamespace="urn:test" elementFormDefault="qualified">
+  <element name="root">
+    <complexType>
+      <sequence>
+        <element name="item">
+          <complexType>
+            <attribute name="target" type="QName"/>
+          </complexType>
+        </element>
+      </sequence>
+    </complexType>
+  </element>
+</schema>
+XSD;
+
+libxml_use_internal_errors(true);
+
+$modern = Dom\XMLDocument::createFromString($xml, LIBXML_NSCLEAN);
+var_dump($modern->schemaValidateSource($xsd));
+
+$legacy = new DOMDocument();
+$legacy->loadXML($xml, LIBXML_NSCLEAN);
+var_dump($legacy->schemaValidateSource($xsd));
+
+foreach (libxml_get_errors() as $error) {
+    echo trim($error->message), PHP_EOL;
+}
+?>
+--EXPECT--
+bool(true)
+bool(true)