Callinng a function after submit

Hello I have a login form I would like to put the login process in a function. here's my code

//this code works
if(isset($_POST['logmein'])){
	$uname = mysql_real_escape_string($_POST['username']);
	$pass = mysql_real_escape_string($_POST['password']);
	
        $rrr = $db->select("SELECT * FROM tbluser WHERE uname = '$uname' and pass = '$pass' LIMIT 1");
	$count=mysql_num_rows($rrr);
	if($count == 0){
		$alert_msg="Invalid username/password.";

	}else{
		$alert_msg="Correct Login";
	}
}

PHP:

But I want to change that code into this:

//function.php
function validateUser($uname,$pass){
       ...query mysql for processing....
}

//loginform.php
include('function.php');
if(isset($_POST['logmein'])){
   //call the function
   validateUser($_POST['username'],$_POST['password']);
}

PHP:

Any suggestion? Please help.. Thanks