<?php
$a = array(
    'a' => NULL,
    'b' => 1,
    'c' => 1
);

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

function arrayMergeIgnoringNull($arr1, $arr2) {
    $new2 = array();
    forEach ($arr2 as $key => $value) {
        if (($value !== NULL) || !isSet($arr1[$key])) {
            $new2[$key] = $value;
        } 
    }
    
    return array_merge($arr1, $new2);
}

$c = arrayMergeIgnoringNull($a, $b);
print_r($c);
?>