Не знаю, правильно ли я вас понял, но в любом случае попробуйте:
HTML:
<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>
jQuery:
$("#submit").hide();
$.post("checkUsername.php", { username: $("#username").val(), password: $("#password").val() }, function(result) {
if(result == "true") { $("#submit").fadeIn(fast); }
});
Php (checkUsername.php):
if(mysql_num_rows(mysql_query("SELECT username,password FROM table WHERE username LIKE '". mysql_real_escape_string($_POST['username']) . "',password LIKE '".mysql_real_escape_string($_POST['password'])."'")))
{
echo "true";
}
else
{
echo "false";
}