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

if(mysql_query("SELECT * FROM users WHERE username='$username' AND password='$password'",$con)){
session_start();
$_SESSION["username"]=$username;
$_SESSION["password"]=$password;
header('location:admin.html');
}
else{
echo "Login Failed.<a href=index.html>Re Login</a";
}

<form method="post" id="loginform" action="validate.php">
<table>
    <tr>
        <td>
            <label>Username :</label>
        </td>
        <td>
            <input type="text" name="username"/>
        </td>
    </tr>
<br>
    <tr>
        <td>
            <label>Password :</label>
        </td>
        <td>
            <input type="password" name="password"/>
        </td>
    </tr>
<br>
    <tr>
        <td>
        <input type="submit" id="login" value="Log In" class="btn btn-primary"/>
        </td>
        <td>
            <input type="reset" id="reset" value="Reset" class="btn btn-primary"/>
        </td>
    </tr>
</table>