Fix GH-17198: SplFixedArray assertion failure with get_object_vars

Because the properties table contains both a numeric index and a string
index that map to 0 in a symbol table, this causes an assertion failure.
Looking at the manual page of get_object_vars(), it seems that only real
properties must be included. Given that SplFixedArray's elements are not
accessible like properties, they should be excluded. This restores PHP
8.3 behaviour. The reason that this didn't cause problems on 8.3 is
because it used a different handler (get_properties).

Closes GH-17206.

NEWS

@@ -75,6 +75,10 @@ PHP                                                                        NEWS
     on SO_SNDTIMEO/SO_RCVTIMEO for socket_set_option().
     (David Carlier)
 
+- SPL:
+  . Fixed bug GH-17198 (SplFixedArray assertion failure with get_object_vars).
+    (nielsdos)
+
 - Streams:
   . Fixed bug GH-17037 (UAF in user filter when adding existing filter name due
     to incorrect error handling). (nielsdos)

ext/spl/spl_fixedarray.c

@@ -239,10 +239,14 @@ static HashTable* spl_fixedarray_object_get_properties_for(zend_object *obj, zen
 	zval *const elements = intern->array.elements;
 	HashTable *ht = zend_new_array(size);
 
-	for (zend_long i = 0; i < size; i++) {
-		Z_TRY_ADDREF_P(&elements[i]);
-		zend_hash_next_index_insert(ht, &elements[i]);
+	/* The array elements are not *real properties*. */
+	if (purpose != ZEND_PROP_PURPOSE_GET_OBJECT_VARS) {
+		for (zend_long i = 0; i < size; i++) {
+			Z_TRY_ADDREF_P(&elements[i]);
+			zend_hash_next_index_insert(ht, &elements[i]);
+		}
 	}
+
 	if (source_properties && zend_hash_num_elements(source_properties) > 0) {
 		zend_long nkey;
 		zend_string *skey;

ext/spl/tests/gh17198.phpt

@@ -0,0 +1,17 @@
+--TEST--
+GH-17198 (SplFixedArray assertion failure with get_object_vars)
+--FILE--
+<?php
+#[AllowDynamicProperties]
+class MySplFixedArray extends SplFixedArray {
+}
+$array = new MySplFixedArray(2);
+$array->{0} = [];
+var_dump(get_object_vars($array));
+?>
+--EXPECT--
+array(1) {
+  [0]=>
+  array(0) {
+  }
+}