Logging In via AJAX/MySql/PHP
<?php
mysql_connect('localhost', '', '');
mysql_select_db('database');
if(isset($_POST['submit'])){
    mysql_query("SELECT username,password FROM table WHERE username='".$_POST['username']."',password='".$_POST['password']."'")
}
?>

<form method="post">
Username <input type="text" name="username" />
Password <input type="password" name="password" />
<input type="submit" name="submit" value="Login" />
</form>

<form method="post">
     Username <input type="text" id="username" name="username" />
     Password <input type="password" id="password" name="password" />
    <input type="submit" name="submit" id="submit" value="Login" />
 </form>

$("#submit").hide();
    $.post("checkUsername.php", { username: $("#username").val(), password: $("#password").val() }, function(result) {
 if(result == "true") { $("#submit").fadeIn(fast); }

 });

if(mysql_num_rows(mysql_query("SELECT username,password FROM table WHERE username LIKE '". mysql_real_escape_string($_POST['username']) . "',password LIKE '".mysql_real_escape_string($_POST['password'])."'")))
    {
       echo "true";
    }
    else
    {

    echo "false";
    }