Add test for bug #55688

ext/session/tests/bug55688.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Bug #55688 (Crash when calling SessionHandler::gc())
+--SKIPIF--
+<?php include('skipif.inc'); ?>
+--INI--
+html_errors=0
+session.save_handler=files
+--FILE--
+<?php
+ini_set('session.save_handler', 'files');
+$x = new SessionHandler;
+$x->gc(1);
+?>
+--EXPECTF--
+Warning: SessionHandler::gc(): Parent session handler is not open in %s on line %d