问如何使用SQL注入利用我的PHP代码

EN

<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<p><font face = "Arial" size="4" color="#6b8e23">Login Menu</font></p>
<input type="text" name="username" placeholder="Username" />
<div><input type="password" name="password" placeholder="password" /></div>
<input type="submit" name="Login" value="Login"></input>

<?php
// connect to database
$host = "localhost";
$username = 'root';
$password = '';
$dbname = "accounts";
$con = mysqli_connect($host, $username, $password, $dbname);

//check to ensure connection exists !DEBUG!
if (!$con) {
    die("Connection Failed: " . mysqli_connect_error());
    ?>
    <p><font face = "Arial" size="4" color="#ff4500">No Connection</font></p>
    <?php
} else {
    ?>
    <p><font face = "Arial" size="4" color="#ff4500">Connection established with sql server </font></p>
<?php
}

//if the form has been submitted
if (isset($_POST['Login']))
{
    //user login details
    $user = $_POST['username'];
    $pass = $_POST['password'];

    if($user != "" && $pass != "")
    {
        $sql_query = "SELECT count(*) as cntUser FROM users where usernames='".$user."' AND passwords='".$pass."'";

        $result = mysqli_query($con, $sql_query);
        $row = mysqli_fetch_array($result);
        $count = $row['cntUser'];

        if($count > 0)
        {
            $_SESSION['username'] = $user;
            ?>
            <p><font face = "Arial" size="4" color="#ff4500">User Found</font></p>
            <?php
        } else {
            ?>
            <p><font face = "Arial" size="4" color="#ff4500">No User by that name</font></p>
            <?php
        }
    }
}
?>