
 <?php

$a = array(
    'a' => NULL,
    'b' => 1,
    'c' => 1
);
 
$b = array(
    'a' => 1,
    'b' => NULL,
    'c' => 1    
);
 
$b  = array_filter($b);
$a  = array_filter($a);
$c = array_merge($a, $b);
var_dump($c);