Возможна комбинация AJAX
и jQuery
HTML:
<select name="foo" id="foo">
<option value="1">one</option>
<option value="2">two</option>
<option value="3">three</option>
<option value="4">four</option>
</select>
JavaScript / jQuery:
$.ajax({
url: "resource.php",
success: function(d){
$('#foo').append('<option value="' + d.value + '">' + d.description + '</option>');
}
});