php - How can I work with Paypal sandbox mode in CI Merchant -

i using following code paypal express.

$settings = array(             'username' => 'xxxx',             'password' => 'xxx',             'signature' => 'xxxx',             'test_mode' => true         ); 

while using above code, following error message shown

merchant_response object (     [_status:protected] => failed     [_message:protected] => security header not valid     [_reference:protected] =>      [_data:protected] =>      [_redirect_url:protected] =>      [_redirect_method:protected] =>     [_redirect_message:protected] =>      [_redirect_data:protected] =>  ) 

but, if changed test_mode false, working fine (redirect paypal payment page). can 1 tell me how can work test_mode?

i created api using following steps

 profile -> selling tools -> selling online – api access -> update ->  request api credentials – request api signature -> agree , submit  

while creating api, specified sandbox mode?

hello should use environment , set sandbox that

$settings = array(             'username' => 'xxxx',             'password' => 'xxx',             'signature' => 'xxxx',             'test_mode' => true,             'environment'=>'sandbox' );