diff --git a/Zend/tests/gh21504.inc b/Zend/tests/gh21504.inc
new file mode 100644
index 0000000000000..71dc5c2e61f64
--- /dev/null
+++ b/Zend/tests/gh21504.inc
@@ -0,0 +1,3 @@
+<?php
+
+'1234' |> var_dump(...);
diff --git a/Zend/tests/gh21504.phpt b/Zend/tests/gh21504.phpt
new file mode 100644
index 0000000000000..5fd9eaff91ab6
--- /dev/null
+++ b/Zend/tests/gh21504.phpt
@@ -0,0 +1,12 @@
+--TEST--
+GH-21504: Incorrect RC-handling for ZEND_EXT_STMT op1
+--FILE--
+<?php
+
+$php_escaped = getenv('TEST_PHP_EXECUTABLE_ESCAPED');
+$cmd = $php_escaped . ' -n -e ' . escapeshellarg(__DIR__ . '/gh21504.inc');
+echo shell_exec($cmd);
+
+?>
+--EXPECT--
+string(4) "1234"
diff --git a/Zend/zend_compile.c b/Zend/zend_compile.c
index ee83ee75ff6ea..ab6f2fb1e98fc 100644
--- a/Zend/zend_compile.c
+++ b/Zend/zend_compile.c
@@ -1950,6 +1950,9 @@ static void zend_do_extended_stmt(znode* result) /* {{{ */
 
 	opline->opcode = ZEND_EXT_STMT;
 	if (result) {
+		if (result->op_type == IS_CONST) {
+			Z_TRY_ADDREF(result->u.constant);
+		}
 		SET_NODE(opline->op1, result);
 	}
 }
diff --git a/ext/dom/node.c b/ext/dom/node.c
index 19b6c2eeae3b9..7ff5c6615a6ca 100644
--- a/ext/dom/node.c
+++ b/ext/dom/node.c
@@ -2132,7 +2132,7 @@ static void dom_relink_ns_decls_element(HashTable *links, xmlNodePtr node)
 
 				ns->_private = attr;
 				if (attr->prev) {
-					attr->prev = attr->next;
+					attr->prev->next = attr->next;
 				} else {
 					node->properties = attr->next;
 				}
diff --git a/ext/dom/tests/modern/xml/gh21548.phpt b/ext/dom/tests/modern/xml/gh21548.phpt
new file mode 100644
index 0000000000000..55299c8d6e679
--- /dev/null
+++ b/ext/dom/tests/modern/xml/gh21548.phpt
@@ -0,0 +1,17 @@
+--TEST--
+GH-21548 (Dom\XMLDocument::C14N() emits duplicate xmlns declarations after setAttributeNS())
+--CREDITS--
+Toon Verwerft (veewee)
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+
+$doc = Dom\XMLDocument::createFromString('<root xmlns="urn:a" attr="val"/>');
+$doc->documentElement->setAttributeNS("http://www.w3.org/2000/xmlns/", "xmlns:ns1", "urn:a");
+
+echo $doc->C14N() . PHP_EOL;
+
+?>
+--EXPECT--
+<root xmlns="urn:a" xmlns:ns1="urn:a" attr="val"></root>