Question

I have the following array below, doing an experiment on union operator.

$a = array(1,2,3,6);

$b = array(1, 2, 3, 4, 5);

$c =  $b + $a;

$c seems to be 1,2,3,4,5

Thus I assume array $b is check against array $a. 6 doesn’t exist in $b, so it doesn’t add to it.

Now, if I were to switch it to
$c = $a + $b, I would expect (1,2,3,6) only.

However, the actual output is (1,2,3,6,5).

Can anyone explain this?

Answer 1

I think you misunderstood with how + operator works with array.
When + is used for array it basically merges 2 arrays by keys not values

For example. I’ll explain why $c has difference result than you expected.

# $a = array(1,2,3,6);
$a = array(
  0 => 1,
  1 => 2,
  2 => 3,
  3 => 6
);
# $b = array(1, 2, 3, 4, 5);
$b = array(
  0 => 1, 
  1 => 2, 
  2 => 3, 
  3 => 4, 
  4 => 5
);

// the + operator simply do this
$c[0] = $a[0] + $b[0]; // yield 1
...
...
$c[3] = $a[3] + $b[3]; // yield 6

$c; // yield array(1,2,3,6,5)