How to evaluate a SQL server string with a PHP function?

I have the following PHP function that should evaluate a string, to check "if it exists in the database or not" the string comes to the $row variable as an email format. How can I evaluate the string? Please help and thank you!

<?php

if (isset($_POST['submit'])) {
    $firstName = $_POST['firstName'];
    $lastName = $_POST['lastName'];
    $username = $_POST['username'];
    checkUsername($firstName, $lastName, $username);
    }

function checkUsername($firstName, $lastName, $username){
include 'connect_sql.php'; 

        $sql = "SELECT UserName FROM [Membership].[dbo].[Center] WHERE UserName='$username'";
        $stmt = sqlsrv_query( $conn, $sql );
        $row = sqlsrv_fetch_array( $stmt, SQLSRV_FETCH_ASSOC);

        if(sqlsrv_num_rows($row) > 0 ) {              
            echo "<div id='loginmsg'>Member already exist</div>";
        }

        else {
            echo "<div id='loginmsg'>Member is new</div>";
        }
    }
?>

I tried the following (sqlsrv_num_rows($row) > 0 ) having fetch a row but it jumps to else condition. I tried running the sql query directly at the database and it works fine. I'm not sure what's wrong with the code or how to handle when it finds a row to echo that the record exist or not. Please help!