php - Having error with the following line

first error : Strict Standards: Only variables should be passed by reference in /home/user/public_html/ref/hhhhh_hhhhh/index.php on line 15

After clicking submit button :

Warning: Cannot modify header information - headers already sent by (output started at /home/user/public_html/ref/hhhhh_hhhhh/index.php:3) in /home/user/public_html/ref/hhhhh_hhhhh/index.php on line 28

require_once 'jsonRPCClient.php';
$api_key = 'apikey';
$api_url = 'url';
$client = new jsonRPCClient($api_url);
$campaigns = $client->get_campaigns(
    $api_key,
    array (
        # find by name literally

        'name' => array ( 'EQUALS' => 'test' )
    )
);
$CAMPAIGN_ID = array_pop(array_keys($campaigns));
if(isset($_POST['submit'])) 
{
    $result = $client->add_contact(
    $api_key,
    array (
        'campaign'  => $CAMPAIGN_ID,

        'name'      => 'Test',
        'email'     => 'test@test.test',
    )
    );
$cid = "infod";
$site_url = $cid.".pokemon.com";    
header("Location: http://$site_url") ;
}

?>