<?php

    $a = array(
		'a' => NULL,
		'b' => 1,
		'c' => 1,
		'd' => NULL
	);
	
	$b = array(
		'a' => 1,
		'b' => NULL,
		'c' => 1,   
		'd' => NULL,
	);
	
	$c = array();
	foreach($a as $key => $val)
	{
		if($key == NULL && $b[$key] == NULL)
		{
			$c[$key] = $val;
		} else if($key != NULL && $b[$key] == NULL) {
			$c[$key]= $val;
		} else if($key != NULL && $b[$key] != NULL) {
			$c[$key]= $b[$key];
		} else {
			$c[$key]= $b[$key];
		}
	}
	var_dump($c);