<?php
$a = array('a','b');
$b = array('c','d');
$c = array('e','f');
$a2 = array('a2','b2');
$b2 = array('c2','d2');
$c2 = array('e2','f2');
$a3 = array('a3','b3');
$b3 = array('c3','d3');
$c3 = array('e3','f3');
$a4 = array('a4','b4');
$b4 = array('c4','d4');
$c4 = array('e4','f4');
$aaa = array_push($a,$b);
$aaa2 = array_merge($c,$a2);
$aaa3 = $b2 + $c2;

$d = null;
$d2 = null;
$d3 = null;
$d4 = null;
$aaa4 = array_merge($a3,$d);
$aaa5 = array_merge($d2,$b3);

$aaa6 = array_push($c3,$d3);
$aaa7 = array_push($d4, $a4);

debug:
alt

summary:

array_push(null,array('not null')) => null
array_push(array('not null'),null) => not null
array_merge(null,array('not null')) => null
array_merge(array('not null'),null) => null