Executing a SQL Server Stored Procedure with parameters using PDO in php

I seem to be having great difficulty in executing a ms sql server stored procedure that contains parameters, I am able to execute either simple select statements or stored procedures without parameters but as soon as I try to bind parameters i get the message :

"An error occured: SQLSTATE[HY000]: General error: 102 General SQL Server error: Check messages from the SQL Server [102] (severity 15) [(null)]"

code from php is:

    $pdo = new PDO($dsn, $user, $password);

    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    try {
    $sql = $pdo->prepare("EXEC [contractor].[sync_licence] (?)");
    $params = 5;
    $sql->bindParam(1, $params, PDO::PARAM_STR|PDO::PARAM_INPUT_OUTPUT, 4000);
    $sql->execute();
    $results = $sql->fetchAll();
    var_dump($results);
    } catch (PDOException $e) {
    echo 'An error occured: ' . $e->getMessage();
    }
Copy code

Can anybody help with where I am going wrong?