Username or email availability or any other types of availability check we can do in this way. When user type on a text box an ajax request has made and check what if its available or not.  we only required jquery.js

HTML CODE:

<input name="username" type="text" id="username" />
<label id="user_msg" style="color:#CC0000;"></label>

Javascirpt:

$(document).ready(function() {

		var timeOut = null;	// this used for hold few seconds to made ajax request

		var loading_html = '<img src="ajax-loader.gif" />'; // just an loading image or we can put any texts here

		//when button is clicked
		$('#username').keyup(function(e){

			// when press the following key we need not to make any ajax request, you can customize it with your own way
			switch(e.keyCode)
			{
				//case 8:   //backspace
				case 9:		//tab
				case 13:	//enter
				case 16:	//shift
				case 17:	//ctrl
				case 18:	//alt
				case 19:	//pause/break
				case 20:	//caps lock
				case 27:	//escape
				case 33:	//page up
				case 34:	//page down
				case 35:	//end
				case 36:	//home
				case 37:	//left arrow
				case 38:	//up arrow
				case 39:	//right arrow
				case 40:	//down arrow
				case 45:	//insert
				//case 46: 	//delete
					return;
			}
			if (timeOut != null)
			    clearTimeout(ajaxCallTimeoutID);
			timeOut = setTimeout(is_available, 1000);  // delay delay ajax request for 1000 milliseconds
			$('#user_msg').html(loading_html); // adding the loading text or image
		});
  });
function is_available(){
	//get the username
	var username = $('#username').val();

	//make the ajax request to check is username available or not
	$.post("availability.php", { username: username },
	function(result)
	{
		if(result != 0)
		{
			$('#user_msg').html('Not Available');
		}
		else
		{
			$('#user_msg').html('<span style="color:#006600;">Available</span>');
		}
	});

}

PHP: Put your way to searching, you can make any array search or you can check it from database or flat file. I check it from database here is the code

<?php
mysql_connect('localhost', 'root', '');
mysql_select_db('db_name');
$username = mysql_real_escape_string($_POST['username']);
$result = mysql_query('select username from user_table where username = "'. $username .'"');
$cnt = mysql_num_rows($result);
print($cnt);
?>

Its work with me very well 🙂