Merging arrays even if they don't exist with PHP

I currently have 3 arrays, $A, $B and $C that I need to merge into a singe array called $allArrays. I know I can do this with array_merge ``` $allArrays = array_merge($A,$B, $C); ``` But sometimes not all 3 arrays will exist. Depending on if a user of this site I am working on activates an option that would create an array, I sometimes might only end up with 1 of the three, 2 of the three or maybe all 3. So if user 1 actives an option that creates array $A and $B, while user 2 actives an option that creates array $A and $C, regardless one, both, or all three need to be merged to $allArrays. How can I do this? Currently, if I don't active all three arrays, allArrays comes back blank and empty, 0. I feel like I could easily achieve this with multiple if statements to see if all arrays exists but I feel like that's a bit excessive. I am new with PHP so looking into what to search for has been difficult. Thanks for any help!