在php中调用存储过程时遇到问题

Question

我是PHP新手。我现在要做的就是从MySQL服务器调用存储过程。我能够使用标准的SELECT语句成功查询数据库，但是当我尝试调用存储过程时，它永远不会工作。如果我将$ sql换成标准的SELECT * FROM表，它就可以了。我到处寻找。但无法弄清楚该怎么做。我哪里错了？

    <?php
    $servername = server;
    $username = username;
    $password = password;
    $dbname = databasename;

    $conn = new mysqli($servername, $username, $password, $dbname);

    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }
    else {
        echo "It worked!<br/><br/>";
    }

    if(mysqli_ping($conn) != null) {
        echo "Connection is open!<br/><br/>";

        $sql = "CALL GetUserTypes";

        $result = $conn->query("$sql");


        if ($result != null) {
            $i = 0;
            $row = mysqli_fetch_row($result);

            while ($row = $result->fetch_assoc()) {
                echo '<p>' . $row['id']. "      " . $row['type'] . '</p>';
                $i++;
            }
        }
        elseif (is_null($result)) {
            echo "Something broke...<br/>";
        }
        else {
            echo "Query did not work...<br/>";
        }

        $con->close();

    else {
        echo "No connection...";
    }
    ?>