Php arraymerge vs arrayreplace vs plus aka union

array_merge vs array_replace vs + (plus aka union) in PHP

If you wrote some PHP, most likely you have used array_merge here and there for your arrays. You may have met array_replace function, introduced in PHP5.3. Finally, you could notice + operator (aka union).

Since all 3 do similar thing, and the docs don’t quite describe the difference between them, here’s a nice image of it

There are a few things to consider:

1. array_merge and array_replace work just the same for keyed (associative) elements, so they can be used interchangeably:

    // associative arrays
    array_replace($a, $b) === array_merge($a, $b)
   

2. array_replace and + do the opposite always:

    // numeric arrays
    array_replace($a, $b) === $b + $a
   
    // associative arrays
    array_replace($a, $b) == $b + $a // equal, but not identical ===
   

3. array_merge behaves differently to the other 2 with numeric arrays:

    // numeric arrays
    array_replace($a, $b) != array_merge($a, $b)
   

That being said, we could think of the + operator as kinda redundant, since the same can be achieved with array_replace function.

However, there are cases, where it comes in handy: say you have an $options array being passed to a function/method, and there are also defaults to be used as fallback:

// we could do it like this
function foo(array $options)
{
   $defaults = ['foo' => 'bar'];
   
   $options = array_replace($defaults, $options);

   // ...
}

// but + here might be way better:
function foo(array $options)
{
   $options += ['foo' => 'bar'];

   // ...
}