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

<input type='text' id='username'> <input type='button' id='check_username_availability' value='Check Availability'>  
<div id='username_availability_result'></div>

PHP

//connect to database  
mysql_connect('host', 'user', 'pass');  
mysql_selec_db('database_name');  

//get the username  
$username = mysql_real_escape_string($_POST['username']);  

//mysql query to select field username if it's equal to the username that we check '  
$result = mysql_query('select username from users where username = "'. $username .'"');  

//if number of rows fields is bigger them 0 that means it's NOT available '  
if(mysql_num_rows($result)>0){  
    //and we send 0 to the ajax request  
    echo 0;  
}else{  
    //else if it's not bigger then 0, then it's available '  
    //and we send 1 to the ajax request  
    echo 1;  
} 

JQuery

$(document).ready(function() {  

        //the min chars for username  
        var min_chars = 3;  

        //result texts  
        var characters_error = 'Minimum amount of chars is 3';  
        var checking_html = 'Checking...';  

        //when button is clicked  
        $('#check_username_availability').click(function(){  
            //run the character number check  
            if($('#username').val().length < min_chars){  
                //if it's bellow the minimum show characters_error text '  
                $('#username_availability_result').html(characters_error);  
            }else{  
                //else show the cheking_text and run the function to check  
                $('#username_availability_result').html(checking_html);  
                check_availability();  
            }  
        });  

  });  

//function to check username availability  
function check_availability(){  

        //get the username  
        var username = $('#username').val();  

        //use ajax to run the check  
        $.post("check_username.php", { username: username },  
            function(result){  
                //if the result is 1  
                if(result == 1){  
                    //show that the username is available  
                    $('#username_availability_result').html(username + ' is Available');  
                }else{  
                    //show that the username is NOT available  
                    $('#username_availability_result').html(username + ' is not Available');  
                }  
        });  

}  

That's it.

You will need to change only :

host(mysql db host, usualy localhost)
user(mysql db username)
pass(mysql db password)
database_name(mysql db name)
username(mysql db table field)
users(mysql db table name)