This is just a reminder for me because I always struggle to find a good documentation:

When dealing with simple arrays without numeric indexes in PHP, there are 3 importants things to keep in mind:

1. The arguments order passed to the function or used with the + sign is important;
2. array_merge and array_replace are interchangeable since their results are identical;
3. the + sign gives the same results as array_merge and array_replace but the arguments order must be reverse;

So performance wise, the + sign is better but beware or your arguments orders!!

And here’s a little demo to make it clear to you