Ajax Sample

<?php
	$usuarios = array(
		array('login' => 'fulano', 'email' => 'fulano@exemplo.com'),
		array('login' => 'beltrano', 'email' => 'beltrano@exemplo.com'),
		array('login' => 'siclano', 'email' => 'siclano@exemplo.com')
	);

	if($_POST) {
		$login   = $_POST['login'];
		$email   = $_POST['email'];

		$encontrado = array();

		foreach($usuarios as $usuario) {
			if($usuario['login'] === $login)
				$encontrado['login'] = true;
			if($usuario['email'] === $email)
				$encontrado['email'] = true;
		}
		header('Content-type: application/json');

		$json = array();

		if($encontrado) {
			$json['status']  = false;
			$json['message'] = "Login ou email em uso.";
			$json['data']    = $encontrado;
		} else {
			$json['status']  = true;
			$json['message'] = "Cadastro efetuado com sucesso.";
		}

		exit(json_encode($json));
	}
?>

<html>
	<head>
		<title>Teste</title>
	</head>
	<body>
		<form id="myForm" method="post">
			<label>
				Login
				<input name="login">
			</label>
			<label>
				Email
				<input name="email" type="email">
			</label>
			<input type="submit" value="Enviar">
		</form>
		<script src="https://code.jquery.com/jquery-2.1.4.min.js"></script>
		<script>
			jQuery(function($) {
				var $myForm = $('#myForm');
				$myForm.submit(function() {
					$.post(window.location.href, $myForm.serialize(), function(response) {
						if(response.status) {
							$('input').css('border-color', '');
						} else {
							for(field in response.data) {
								$('[name=' + field + ']').css('border-color', 'red');
							}
						}

						alert(response.message);
					});
					return false;
				});
			});
		</script>
	</body>
</html>