diff --git a/Zend/Optimizer/block_pass.c b/Zend/Optimizer/block_pass.c
index 02c28ead33e1..3775f8165c3e 100644
--- a/Zend/Optimizer/block_pass.c
+++ b/Zend/Optimizer/block_pass.c
@@ -648,7 +648,7 @@ static void zend_optimize_block(zend_basic_block *block, zend_op_array *op_array
 					} else if (opline->op1_type == IS_TMP_VAR &&
 					           !zend_bitset_in(used_ext, VAR_NUM(opline->op1.var))) {
 						src = VAR_SOURCE(opline->op1);
-						if (src) {
+						if (src && (src->op1_type != IS_VAR)) {
 							if (src->opcode == ZEND_BOOL_NOT) {
 								VAR_SOURCE(opline->op1) = NULL;
 								COPY_NODE(opline->op1, src->op1);
@@ -692,7 +692,7 @@ static void zend_optimize_block(zend_basic_block *block, zend_op_array *op_array
 					           (!zend_bitset_in(used_ext, VAR_NUM(opline->op1.var)) ||
 					            opline->result.var == opline->op1.var)) {
 						src = VAR_SOURCE(opline->op1);
-						if (src) {
+						if (src && (src->op1_type != IS_VAR)) {
 							if (src->opcode == ZEND_BOOL ||
 							    src->opcode == ZEND_QM_ASSIGN) {
 								VAR_SOURCE(opline->op1) = NULL;
diff --git a/ext/opcache/tests/gh21691.phpt b/ext/opcache/tests/gh21691.phpt
new file mode 100644
index 000000000000..3926b76c7a12
--- /dev/null
+++ b/ext/opcache/tests/gh21691.phpt
@@ -0,0 +1,20 @@
+--TEST--
+GH-21691 (OPcache CFG optimizer breaks reference returns with JMPZ/JMPZ_EX)
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+--EXTENSIONS--
+opcache
+--FILE--
+<?php
+
+function &getData() {
+    return $x;
+}
+
+$data = &getData() && isset($data['key']);
+var_dump($data);
+
+?>
+--EXPECT--
+NULL
diff --git a/ext/standard/tests/serialize/serialize_array_ref_reentrancy.phpt b/ext/standard/tests/serialize/serialize_array_ref_reentrancy.phpt
new file mode 100644
index 000000000000..490fa4f6c3b0
--- /dev/null
+++ b/ext/standard/tests/serialize/serialize_array_ref_reentrancy.phpt
@@ -0,0 +1,21 @@
+--TEST--
+serialize(): a by-reference __serialize() that grows the array being walked must not free it
+--FILE--
+<?php
+class G {
+    public $ref;
+    public function __serialize(): array {
+        for ($i = 0; $i < 128; $i++) {
+            $this->ref[] = 'x' . $i;
+        }
+        return ['d' => 1];
+    }
+}
+$g = new G();
+$inner = [$g, 'tail'];
+$g->ref = &$inner;
+$top = [&$inner];
+var_dump(serialize($top));
+?>
+--EXPECT--
+string(59) "a:1:{i:0;a:2:{i:0;O:1:"G":1:{s:1:"d";i:1;}i:1;s:4:"tail";}}"
diff --git a/ext/standard/var.c b/ext/standard/var.c
index 3137a270f661..d79b8941634d 100644
--- a/ext/standard/var.c
+++ b/ext/standard/var.c
@@ -1303,13 +1303,17 @@ static void php_var_serialize_intern(smart_str *buf, zval *struc, php_serialize_
 				zend_release_properties(myht);
 				return;
 			}
-		case IS_ARRAY:
+		case IS_ARRAY: {
 			smart_str_appendl(buf, "a:", 2);
 			myht = Z_ARRVAL_P(struc);
+			bool rcn = !is_root && (in_rcn_array || GC_REFCOUNT(myht) > 1);
+			GC_TRY_ADDREF(myht);
 			php_var_serialize_nested_data(
 				buf, struc, myht, zend_array_count(myht), /* incomplete_class */ false, var_hash,
-				!is_root && (in_rcn_array || GC_REFCOUNT(myht) > 1));
+				rcn);
+			GC_TRY_DTOR_NO_REF(myht);
 			return;
+		}
 		case IS_REFERENCE:
 			struc = Z_REFVAL_P(struc);
 			goto again;