将第三方api与oauth客户端集成时出现错误HTTP / 1.1 500内部服务器错误

I integrating 3rd party api with ouath 1.o in php

http://api.seatseller.travel/docs/interface.html i getting problem for one api that need send parameter using post method here is my code

$key = 'xxxxxxxxx';
$secret = 'xxxxxxxxxxxxx';
$options = array('consumer_key' => $key, 'consumer_secret' => $secret);
OAuthStore::instance("2Leg", $options);
$method = "POST";
$pass = array(
    'name' => "tempuser",
    'mobile' => "5454545454",
    'title' => "Mr",
    'email' => "red@gamil.com",
    'age' => "45",
    'gender' => "male"
);
$inv = array(
    'seatname' => 1,
    'fare' => 1500.00,
    'ladiesSeat' => false,
    'passenger' => $pass
);
$params = array(
    'availableTripID' => 555555,
    'inventoryItems' => $inv
);
$out = '';
$err = '';
$result = null;
try {
    $request = new OAuthRequester($api_url, $method, $params);
    $result = $request->doRequest();
    $response = htmlspecialchars($result['body']);
    $out .= "<br/><br/>".$response."<br/><br/>";
}
catch(Exception $e) {
    $err .= "Exception with message : ".$e->getMessage();
}
echo 'Output : <br/><br/>'.$out;
echo '<br/><br/>Error : <br/><br/>'.$err;

when i run this code i am getting this error

-HTTP/1.1 500 Internal Server Error