Jquery and ajax, my function for username checking?
function check_username() {
    var username = $("#username").val();
    $('.loading').fadeIn().delay(100);
    $.post("ajax.php", {
        username: $('#username').val(),
    }, function (response) {
        $('.error, .success').hide();
        setTimeout(function () {
            $('.loading').hide();
            finishAjax('username', response);
        }, 1000);
    });
    return false;
}

function finishAjax(id, response) {
    $('#' + id).after(response).fadeIn(1000);
}

<?php

require('dbc.php');

if (isset($_REQUEST['username'])) {

  $q = $dbc -> prepare("SELECT username FROM accounts WHERE username = ?");
  $q -> execute(array($_REQUEST['username']));

  if (strlen($_REQUEST['username']) < 3) {
    echo '<div class="error">Has to be at least 3 characters</div>';
  }
  elseif ($q -> rowCount() > 0) {
    echo '<div class="error">Username already taken</div>';
  }
  else {
    echo '<div class="success">Username available</div>';
  }
}

?>

$.fn.checkValid = function(options)
{
    var response = function(response) {
        var setClass = '';
        var $span = $(this).data('checkValidTip');
        if ($span)
        {
            $span.remove();
        }

        if (response === undefined) return;

        setClass = (response.valid ? 'valid' : 'invalid');

        var $span = $('<span>' + response.msg + '</span>');
        $(this)
            .data('checkValidTip', $span)
            .after($span);

        $span.hide()
             .fadeIn(1000)[0]
             .className = setClass;
    };


    var ajaxOptions = {
        type: 'GET',
        url: 'ajax.php',
        success: response,
        dataType: 'json'
    };

    this.each(function() {

        var that = this;
        var ajaxRequest = ajaxOptions;
        ajaxRequest.data = {};
        ajaxRequest.data[options.key] = this.value;
        ajaxRequest.context = that

        $.ajax(ajaxRequest);
    });
};

$('#username, #email').blur(function() {
    $(this).checkValid({ key: this.id });
});

<?php
   // Do your sql statements here, decide if input is valid or not

   $arr = array('valid' => $is_valid,
                'msg'   => $error_or_good_msg
               );

  echo json_encode($arr);

 /* For example will output:
   {
      "valid": "false",
      "msg": "<b>Error: Must be at least 2 characters</b>"
    }
  Which can be read directly as response.valid
  or response.msg from within response() function
 */

var validations = {
    username: {
      min_chars: 4,
      max_chars: 10,
      valid_chars: 'qwertyuiopasdfghjklzxcvbnm_-'
    },

    email: {
       regex: /./ //your magic regex here
  }
};