从站点返回时的会话超时

Question

I have an issue with PHP session that times out after a visit to Online Payment Service (OPS). Here's the piece of the code that redirects.

$status['redirectUrl'] = "https://test-confined.payex.com/PxOrderCC.aspx?orderRef=308ae6616c94408bbe13c6b3d29f8392"; // link autogenerated each call. 

header('Location:'.$status['redirectUrl']);
die();

I specify a return url in the setup, but when the OPS redirects the session is gone! The strange thing is if I log on again and do the same procedure the session is not lost after a visit to the OPS.