diff --git a/ext/spl/spl_heap.c b/ext/spl/spl_heap.c
index 19e86b0d80b72..254fbde7b3ff0 100644
--- a/ext/spl/spl_heap.c
+++ b/ext/spl/spl_heap.c
@@ -1167,7 +1167,7 @@ static zend_result spl_heap_unserialize_internal_state(HashTable *state_ht, spl_
 	return SUCCESS;
 }
 
-PHP_METHOD(SplPriorityQueue, __serialize)
+static void spl_heap_serialize_internal(INTERNAL_FUNCTION_PARAMETERS, bool is_pqueue)
 {
 	spl_heap_object *intern = Z_SPLHEAP_P(ZEND_THIS);
 	zval props, state;
@@ -1185,14 +1185,18 @@ PHP_METHOD(SplPriorityQueue, __serialize)
 
 	array_init(return_value);
 
-	ZVAL_ARR(&props, zend_std_get_properties(&intern->std));
-	Z_TRY_ADDREF(props);
+	ZVAL_ARR(&props, zend_array_dup(zend_std_get_properties(&intern->std)));
 	zend_hash_next_index_insert(Z_ARRVAL_P(return_value), &props);
 
-	spl_heap_serialize_internal_state(&state, intern, true);
+	spl_heap_serialize_internal_state(&state, intern, is_pqueue);
 	zend_hash_next_index_insert(Z_ARRVAL_P(return_value), &state);
 }
 
+PHP_METHOD(SplPriorityQueue, __serialize)
+{
+	spl_heap_serialize_internal(INTERNAL_FUNCTION_PARAM_PASSTHRU, true);
+}
+
 PHP_METHOD(SplPriorityQueue, __unserialize)
 {
 	HashTable *data;
@@ -1241,28 +1245,7 @@ PHP_METHOD(SplPriorityQueue, __unserialize)
 
 PHP_METHOD(SplHeap, __serialize)
 {
-	spl_heap_object *intern = Z_SPLHEAP_P(ZEND_THIS);
-	zval props, state;
-
-	ZEND_PARSE_PARAMETERS_NONE();
-
-	if (UNEXPECTED(spl_heap_consistency_validations(intern, false) != SUCCESS)) {
-		RETURN_THROWS();
-	}
-
-	if (intern->heap->flags & SPL_HEAP_WRITE_LOCKED) {
-		zend_throw_exception(spl_ce_RuntimeException, "Cannot serialize heap while it is being modified.", 0);
-		RETURN_THROWS();
-	}
-
-	array_init(return_value);
-
-	ZVAL_ARR(&props, zend_std_get_properties(&intern->std));
-	Z_TRY_ADDREF(props);
-	zend_hash_next_index_insert(Z_ARRVAL_P(return_value), &props);
-
-	spl_heap_serialize_internal_state(&state, intern, false);
-	zend_hash_next_index_insert(Z_ARRVAL_P(return_value), &state);
+	spl_heap_serialize_internal(INTERNAL_FUNCTION_PARAM_PASSTHRU, false);
 }
 
 PHP_METHOD(SplHeap, __unserialize)
diff --git a/ext/spl/tests/SplHeap_serialize_indexed_format.phpt b/ext/spl/tests/SplHeap_serialize_indexed_format.phpt
index c201feb5716f6..21ea74c6bff9b 100644
--- a/ext/spl/tests/SplHeap_serialize_indexed_format.phpt
+++ b/ext/spl/tests/SplHeap_serialize_indexed_format.phpt
@@ -74,9 +74,9 @@ array(2) {
   [0]=>
   array(2) {
     ["flags"]=>
-    UNKNOWN:0
+    string(13) "user_property"
     ["heap_elements"]=>
-    UNKNOWN:0
+    string(13) "user_property"
   }
   [1]=>
   array(2) {
diff --git a/ext/spl/tests/gh20101.phpt b/ext/spl/tests/gh20101.phpt
new file mode 100644
index 0000000000000..ad2399d76c8e6
--- /dev/null
+++ b/ext/spl/tests/gh20101.phpt
@@ -0,0 +1,49 @@
+--TEST--
+GH-20101 (SplHeap/SplPriorityQueue serialization exposes INDIRECTs)
+--FILE--
+<?php
+class CustomHeap extends SplMaxHeap {
+    public $field = 0;
+}
+$heap = new CustomHeap();
+$data = $heap->__serialize();
+var_dump($data);
+
+class CustomPriorityQueue extends SplPriorityQueue {
+    public $field = 0;
+}
+$pqueue = new CustomPriorityQueue();
+$data = $pqueue->__serialize();
+var_dump($data);
+?>
+--EXPECT--
+array(2) {
+  [0]=>
+  array(1) {
+    ["field"]=>
+    int(0)
+  }
+  [1]=>
+  array(2) {
+    ["flags"]=>
+    int(0)
+    ["heap_elements"]=>
+    array(0) {
+    }
+  }
+}
+array(2) {
+  [0]=>
+  array(1) {
+    ["field"]=>
+    int(0)
+  }
+  [1]=>
+  array(2) {
+    ["flags"]=>
+    int(1)
+    ["heap_elements"]=>
+    array(0) {
+    }
+  }
+}