Formulario AJAX simple y rápido.

<?php
	if($_POST){ ?>
		<b>Campo 1:</b> <?=$_POST['campo1']?><br>
		<b>Campo 2:</b> <?=$_POST['campo2']?>
	<? exit;
	}
?>
<script>
	function AJAX(u, c, d){
		var XMLReq = new XMLHttpRequest();
		XMLReq.onreadystatechange = function(){
			if(XMLReq.readyState==4) c(XMLReq.responseText);
		};
		if(d) XMLReq.open('POST', u), XMLReq.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
		else XMLReq.open('GET', u);
		XMLReq.send(d)
	}

	function sendForm(f, c){
		inputElements = f.querySelectorAll('input,textarea,select,button');
		for(i = 0, postVar = ''; i < inputElements.length; postVar += inputElements[i].name+'='+inputElements[i].value+'&', i++);
		AJAX(f.action, c, postVar);
		return false
	}


	function byId(i){
		return document.getElementById(i)
	}

</script>
<form method="post" onsubmit="return sendForm(this, function(h){ byId('status').innerHTML = h })">
	<b>Campo 1:</b><input type="text" name="campo1"><br>
	<b>Campo 2:</b><textarea name="campo2" style="width:300px;height:100px"></textarea>
	<input type="submit" value="Enviar">
</form>
<div id="status"></div>

onsubmit="return sendForm(this, CALLBACK)"