diff --git a/ext/standard/tests/serialize/serialize_as_undef.phpt b/ext/standard/tests/serialize/serialize_as_undef.phpt
new file mode 100644
index 000000000000..77189d315669
--- /dev/null
+++ b/ext/standard/tests/serialize/serialize_as_undef.phpt
@@ -0,0 +1,24 @@
+--TEST--
+serialize() serializing array values as undefined
+--FILE--
+<?php
+class NoSleep {
+    public int $x;
+}
+$x = new NoSleep();
+// TODO: Only do this for serialize($x, ['explicit_undefined' => true])
+echo ($s = serialize($x)), "\n";
+$sCopy = unserialize($s);
+var_export($sCopy);
+echo "\n";
+try {
+    $sCopy->x = 'invalid';
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+O:7:"NoSleep":1:{s:1:"x";U;}
+NoSleep::__set_state(array(
+))
+Cannot assign string to property NoSleep::$x of type int
diff --git a/ext/standard/var.c b/ext/standard/var.c
index e1bc4c5597f4..9c92e9192924 100644
--- a/ext/standard/var.c
+++ b/ext/standard/var.c
@@ -879,7 +879,22 @@ static void php_var_serialize_nested_data(smart_str *buf, zval *struc, HashTable
 		zval *data;
 		zend_ulong index;
 
-		ZEND_HASH_FOREACH_KEY_VAL_IND(ht, index, key, data) {
+		ZEND_HASH_FOREACH_KEY_VAL(ht, index, key, data) {
+			if (Z_TYPE_P(data) == IS_INDIRECT) {
+				data = Z_INDIRECT_P(data);
+			}
+			if (UNEXPECTED(Z_TYPE_P(data) == IS_UNDEF)) {
+				if (Z_TYPE_P(struc) != IS_OBJECT) {
+					continue;
+				}
+				if (!key) {
+					php_var_serialize_long(buf, index);
+				} else {
+					php_var_serialize_string(buf, ZSTR_VAL(key), ZSTR_LEN(key));
+				}
+				smart_str_appendl(buf, "U;", 2);
+				continue;
+			}
 			if (incomplete_class && strcmp(ZSTR_VAL(key), MAGIC_MEMBER) == 0) {
 				continue;
 			}
@@ -1013,7 +1028,20 @@ static void php_var_serialize_intern(smart_str *buf, zval *struc, php_serialize_
 					php_var_serialize_class_name(buf, &obj);
 					smart_str_append_unsigned(buf, zend_array_count(Z_ARRVAL(retval)));
 					smart_str_appendl(buf, ":{", 2);
-					ZEND_HASH_FOREACH_KEY_VAL_IND(Z_ARRVAL(retval), index, key, data) {
+					ZEND_HASH_FOREACH_KEY_VAL(Z_ARRVAL(retval), index, key, data) {
+						if (Z_TYPE_P(_z) == IS_INDIRECT) {
+							_z = Z_INDIRECT_P(_z);
+						}
+						if (UNEXPECTED(Z_TYPE_P(_z) == IS_UNDEF)) {
+							// TODO: Only do this if different from the default?
+							if (!key) {
+								php_var_serialize_long(buf, index);
+							} else {
+								php_var_serialize_string(buf, ZSTR_VAL(key), ZSTR_LEN(key));
+							}
+							smart_str_appendl(buf, "U;", 2);
+							continue;
+						}
 						if (!key) {
 							php_var_serialize_long(buf, index);
 						} else {
@@ -1087,10 +1115,11 @@ static void php_var_serialize_intern(smart_str *buf, zval *struc, php_serialize_
 				myht = zend_get_properties_for(struc, ZEND_PROP_PURPOSE_SERIALIZE);
 				/* count after serializing name, since php_var_serialize_class_name
 				 * changes the count if the variable is incomplete class */
-				count = zend_array_count(myht);
+				count = zend_hash_num_elements(myht);
 				if (count > 0 && incomplete_class) {
 					--count;
 				}
+				// Need to call a different helper from zend_array_count, I guess.
 				php_var_serialize_nested_data(buf, struc, myht, count, incomplete_class, var_hash);
 				zend_release_properties(myht);
 				return;
diff --git a/ext/standard/var_unserializer.re b/ext/standard/var_unserializer.re
index c395f3cb5ca2..0dcc9602586c 100644
--- a/ext/standard/var_unserializer.re
+++ b/ext/standard/var_unserializer.re
@@ -593,21 +593,31 @@ string_key:
 				goto failure;
 			}
 		}
+		// TODO: Assert `d` is not IS_UNDEF
+		// Delete the created property if needed.
 
 		if (!php_var_unserialize_internal(data, p, max, var_hash, 0)) {
 			zval_ptr_dtor(&key);
 			goto failure;
 		}
-
-		if (UNEXPECTED(info)) {
-			if (!zend_verify_prop_assignable_by_ref(info, data, /* strict */ 1)) {
-				zval_ptr_dtor(data);
-				ZVAL_UNDEF(data);
-				zval_dtor(&key);
+		if (UNEXPECTED(Z_TYPE_P(data) == IS_UNDEF)) {
+			if (UNEXPECTED(!obj)) {
+				zend_error(E_WARNING, "Cannot unserialize undefined value in an array");
 				goto failure;
 			}
-			if (Z_ISREF_P(data)) {
-				ZEND_REF_ADD_TYPE_SOURCE(Z_REF_P(data), info);
+			// Don't perform type checks if this is undefined
+			// FIXME delete the newly set property if it isn't a declared property
+		} else {
+			if (UNEXPECTED(info)) {
+				if (!zend_verify_prop_assignable_by_ref(info, data, /* strict */ 1)) {
+					zval_ptr_dtor(data);
+					ZVAL_UNDEF(data);
+					zval_dtor(&key);
+					goto failure;
+				}
+				if (Z_ISREF_P(data)) {
+					ZEND_REF_ADD_TYPE_SOURCE(Z_REF_P(data), info);
+				}
 			}
 		}
 
@@ -852,6 +862,12 @@ static int php_var_unserialize_internal(UNSERIALIZE_PARAMETER, int as_key)
 	return 1;
 }
 
+"U;"	{
+	*p = YYCURSOR;
+	ZVAL_UNDEF(rval);
+	return 1;
+}
+
 "b:0;"	{
 	*p = YYCURSOR;
 	ZVAL_FALSE(rval);