PHP-DB - How to make JQueryUI dialog login form?

$('#loginButton').on('click', function(){ //the id of the submit button = #loginButton
	
	var username = $('#userfield').val();
	var password = $('#passfield').val();
	
	if (username.length == 0 || password.length == 0 || password.length < 6) { // checks if username and pass have been entered
	
		$('#fillFields').text('*Please enter a valid username/password'); //#fillfields = the div where error message should appear in JQuery UI dialog box
	} else {
		
		//$('#formLogin').text('You have successfully logged in').dialog('close'); //Closes dialog box if logged in currently not in use
		$.post('loginProcess.php', {username: username, password: password}, funtion(data){ // sends data to loginProcess.php
		
			if (data == true){ //checks if the loginProcess has returned true (if the username exists or not)
				alert('true'); // just to check if the code works, display an alert message
			}
	}
		
	});
});

<?php

require 'config.php';

//  LOGIN FORM Process

if (user_exists('super')){ //user_exists() = a user defined function in config.php
	return true;
}
?>

function sanitize($data){
	return mysql_real_escape_string(strip_tags($data));
}

function user_exists($username){

	$username = sanitize($username);
	$query = mysql_query("SELECT COUNT(`id`) FROM `users` WHERE `Username` = '$username'");
	return (mysql_result($query, 0) == 1) ? true : false;
}
?>