PHP MYSQL update query not working

<?php

$con = mysql_connect("~-~-~-~","~-~-~-~","~-~-~-~");

if (!$con){
    die('Could not connect: ' . mysql_error());
}

mysql_select_db("~-~-~-~", $con);

$username_update = mysql_query("UPDATE users SET username='" . $_GET['confirm_username'] . "' WHERE username='$my_username'");

$username_select = mysql_query("SELECT username FROM users WHERE username = '" . $_GET['confirm_username'] . "'");

$row = mysql_fetch_array($username_select);

if (!$row = $my_username){
    ?>
        <div id="username_changed_green">
            Username Changed
        </div>
    <?php
}else{
    ?>
        <div id="incorrect_details_username">
            Incorrect Details
        </div>
    <?php
}
mysql_close($con);
?>