IPN for s2member

You should be able to use wp_remote_post() for that task.

wp_remote_post(
     'http://example.com/s2m/?s2member_paypal_notify=1&s2member_paypal_proxy=LocalGate&s2member_paypal_proxy_verification=905e5a3c73480046f4354946788b8ea1'
    ,array(
         'method'     => 'POST'
        ,'timeout'    => 30
        ,'redirect'   => 5
        ,'user-agent' => 'my-app/1.0'
        ,'blocking'   => true
        ,'compress'   => false
        ,'sslverify'  => false
     )
);

Note that all $args (the 2nd argument/array) have default options. You can read more about it in the Codex.