Check if username exists in database with AJAX

Before continuing, SELECT * FROM 'users' WHERE 'username' = '. $_POST['username']; is just ASKING for a SQL Injection. I suggest you use PHP Data objects.

So as I understood I must pass the POST values via $.ajax too, correct? If yes, how I will be able to access them via validation.php file?

Because this is a simple request, I suggest you use JQuery’s method $.post(). Here’s a sample based off of what you’re trying to do.

$.post('validation.php',{username: $('#username').val()}, function(data){
    if(data.exists){
        //tell user that the username already exists
    }else{
        //username doesn't exist, do what you need to do
    }
 }, 'JSON');

jQuery’s post method takes 4 parameters $.post(url, data, callback, datatype). In the example above, we will be posting the username with $('#username').val() to validation.php and expect a JSON response. When the request is finished, the callback function will be executed with data being the response from the request. Because we specified that that response will be JSON, we can access it just like a native object in javascript. Now let’s move to validation.php

Like I stated above, I suggested you use PDO for your database driver. So in this example, I will show you a basic usage of it.

//set the headers to be a json string
header('content-type: text/json');

//no need to continue if there is no value in the POST username
if (!isset($_POST['username'])) {
    exit;
}

//initialize our PDO class. You will need to replace your database credentials respectively
$db = new PDO('mysql:host=DATABASE_HOST;dbname=DATABASE_NAME;charset=utf8mb4', 'DATABASE_USERNAME', 'DATABASE_PASSWORD');

//prepare our query.
$query = $db->prepare('SELECT COUNT(*) FROM users WHERE username = :name');
//let PDO bind the username into the query, and prevent any SQL injection attempts.
$query->bindParam(':name', $_POST['username']);
//execute the query
$query->execute();

//return the JSON object containing the result of if the username exists or not. The $.post in our jquery will access it.
echo json_encode(array('exists' => $query->fetchColumn() > 0));

Now to recap, our jQuery script will post to validation.php where it selects a username from the database. It will return a JSON object that has a key of exists that is a boolean indicating if the username already exists as a row in your database. When the request is complete via jQuery, you can do what you need based off the result of the query.