diff --git a/NEWS b/NEWS
index 103b778aacf47..fa7d93eccbf6d 100644
--- a/NEWS
+++ b/NEWS
@@ -76,6 +76,8 @@ PHP                                                                        NEWS
     (ilutov)
   . Fixed bug GH-16479 (Use-after-free in SplObjectStorage::setInfo()). (ilutov)
   . Fixed bug GH-16478 (Use-after-free in SplFixedArray::unset()). (ilutov)
+  . Fixed bug GH-15911 (Infinite recursion in AppendIterator::append()).
+    (DanielEScherzer)
 
 - Standard:
   . Fixed bug GH-16293 (Failed assertion when throwing in assert() callback with
diff --git a/ext/spl/spl_array.c b/ext/spl/spl_array.c
index dbe110f7c40da..33fcae3abd1f4 100644
--- a/ext/spl/spl_array.c
+++ b/ext/spl/spl_array.c
@@ -100,6 +100,20 @@ static inline HashTable *spl_array_get_hash_table(spl_array_object* intern) { /*
 }
 /* }}} */
 
+bool spl_array_iterator_contains(zval *object, zval *to_find) {
+	zend_object *obj_to_find = Z_OBJ_P(to_find);
+	spl_array_object *intern = Z_SPLARRAY_P(object);
+	// call spl_array_get_hash_table($40)
+	HashTable *ht = spl_array_get_hash_table(intern);
+	zval *iter = NULL;
+	ZEND_HASH_FOREACH_VAL(ht, iter) {
+		if (Z_OBJ_P(iter) == obj_to_find) {
+			return true;
+		}
+	} ZEND_HASH_FOREACH_END();
+	return false;
+}
+
 static inline bool spl_array_is_object(spl_array_object *intern) /* {{{ */
 {
 	while (intern->ar_flags & SPL_ARRAY_USE_OTHER) {
diff --git a/ext/spl/spl_array.h b/ext/spl/spl_array.h
index e3ea2d3a546e7..f6e9eba847c8d 100644
--- a/ext/spl/spl_array.h
+++ b/ext/spl/spl_array.h
@@ -41,5 +41,6 @@ PHP_MINIT_FUNCTION(spl_array);
 
 extern void spl_array_iterator_append(zval *object, zval *append_value);
 extern void spl_array_iterator_key(zval *object, zval *return_value);
+extern bool spl_array_iterator_contains(zval *object, zval *to_find);
 
 #endif /* SPL_ARRAY_H */
diff --git a/ext/spl/spl_iterators.c b/ext/spl/spl_iterators.c
index 976d29283b965..9bf7dce4a2563 100644
--- a/ext/spl/spl_iterators.c
+++ b/ext/spl/spl_iterators.c
@@ -2961,6 +2961,22 @@ PHP_METHOD(AppendIterator, append)
 
 	SPL_FETCH_AND_CHECK_DUAL_IT(intern, ZEND_THIS);
 
+	if (Z_OBJ_P(ZEND_THIS) == Z_OBJ_P(it)) {
+		zend_argument_value_error(1, "must be different than the iterator being appended to");
+		RETURN_THROWS();
+	}
+	if (instanceof_function(Z_OBJCE_P(it), spl_ce_AppendIterator)) {
+		// Make sure that the iterator being appended doesn't contain the
+		// iterator being appended to, GH-15911
+		// call spl_dual_it_from_obj(it.value.obj)
+		spl_dual_it_object *other_iterator = Z_SPLDUAL_IT_P(it);
+		ZEND_ASSERT(other_iterator->dit_type == DIT_AppendIterator);
+		if (spl_array_iterator_contains(&other_iterator->u.append.zarrayit, ZEND_THIS)) {	
+			zend_argument_value_error(1, "must not contain the iterator being appended to");
+			RETURN_THROWS();
+		}
+	}
+
 	if (intern->u.append.iterator->funcs->valid(intern->u.append.iterator) == SUCCESS && spl_dual_it_valid(intern) != SUCCESS) {
 		spl_array_iterator_append(&intern->u.append.zarrayit, it);
 		intern->u.append.iterator->funcs->move_forward(intern->u.append.iterator);
diff --git a/ext/spl/tests/gh15911_included.phpt b/ext/spl/tests/gh15911_included.phpt
new file mode 100644
index 0000000000000..8a852a4ee6f95
--- /dev/null
+++ b/ext/spl/tests/gh15911_included.phpt
@@ -0,0 +1,17 @@
+--TEST--
+GH-15911 (Appending AppendIterator containing AppendIterator to the contained AppendIterator)
+--FILE--
+<?php
+
+$iterator = new AppendIterator();
+$other = new AppendIterator();
+$other->append( $iterator );
+$iterator->append( $other );
+
+?>
+--EXPECTF--
+Fatal error: Uncaught ValueError: AppendIterator::append(): Argument #1 ($iterator) must not contain the iterator being appended to in %s:%d
+Stack trace:
+#0 %s(%d): AppendIterator->append(Object(AppendIterator))
+#1 {main}
+  thrown in %s on line %d
diff --git a/ext/spl/tests/gh15911_self.phpt b/ext/spl/tests/gh15911_self.phpt
new file mode 100644
index 0000000000000..f470ef84df76f
--- /dev/null
+++ b/ext/spl/tests/gh15911_self.phpt
@@ -0,0 +1,15 @@
+--TEST--
+GH-15911 (Infinite recursion when appending AppendIterator to itself)
+--FILE--
+<?php
+
+$iterator = new AppendIterator();
+
+$iterator->append($iterator);
+?>
+--EXPECTF--
+Fatal error: Uncaught ValueError: AppendIterator::append(): Argument #1 ($iterator) must be different than the iterator being appended to in %s:%d
+Stack trace:
+#0 %s(%d): AppendIterator->append(Object(AppendIterator))
+#1 {main}
+  thrown in %s on line %d