<?php

$array = array (
	0 => array ( "id" => "x", "dbfield" => "y"),
	1 => array ( "id" => "quantity", "dbfield" => "QUANTITY_NEW")
);

$dbfield = array_filter(array_map(function($a){
	if($a["id"] === "quantity"){
		return $a["dbfield"];
	}
}, $array));

print_r($dbfield);