Código PHP:
<html>
<head>
<script type="text/javascript">
function agregar()
{
var o = document.createElement('input');
o.type = "text";
o.name = "lalala";
o.value = "";
document.getElementById('padre').appendChild(o);
}
</script>
</head>
<body>
<div id="padre"></div>
<input type="button" value="Agregar" onclick="agregar()">
</body>
</html>