$a1=["red","green"];
$a2=["blue","yellow"];
print_r(array_merge($a1,$a2));

$a3=["a"=>"red","b"=>"green"];
$a4=["c"=>"blue","b"=>"yellow"];
print_r(array_merge($a3,$a4));

$a=array(3=>"red",4=>"green");
print_r(array_merge($a));