问PHP登录页面为空，而不是回显结果

EN

<form id="login" action="redirect.php" method="post"> <!--This is the form for logging in.-->    
<fieldset id="inputs">
        <input type="hidden" name="ac" value="log"> <!--This value is a "random" value to post so that an if statement will be entered in select.php-->
        <input id="username" name="username" type="text" placeholder="Username" autofocus required>   
        <input id="password" name="password" type="password" placeholder="Password" required>
    </fieldset>

    <fieldset id="actions">
        <input type="submit" id="submit" value="Log in">    
    </fieldset>

<?php
include 'config.php';

$username = $_POST['username'];
$password = $_POST['password'];

function SignIn()
{
session_start();   //starting the session for user profile page
if(!empty($username))   //check to see if the username is empty or not from login.php
{
    $query = mysqli_query($con, "SELECT *  FROM employees where username = ".$username." AND password = ".$password) or die(mysql_error());
    $row = mysqli_fetch_array($query) or die(mysql_error());
    if(!empty($row['username']) AND !empty($row['password']))
    {
        $_SESSION['username'] = $row['password'];
        echo "SUCCESSFULLY LOGGED IN!";

    }
    else
    {
        echo "YOU ENTERED WRONG ID OR PASSWORD...";
    }
}
}
if(isset($_POST['submit']))
{
    SignIn();
}

?>

<?php
/* Database credentials. */
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'myusername');
define('DB_PASSWORD', 'mypassword');
define('DB_NAME', 'ropepart_techportal');

/* Attempt to connect to MySQL database */
$con = mysqli_connect(DB_SERVER, DB_USERNAME, DB_PASSWORD, DB_NAME);

// Check connection
if($con === false){
    die("ERROR: Could not connect. " . mysqli_connect_error());
}
?>