Comparing 2 arrays IF and ONLY IF Array_A doesn't have values found in Array_B?

I need to compare 2 arrays: Array_A and Array_B. I then want to assign to Array_A the values found in Array_B (without deleting what’s already in Array_A) IF and ONLY IF the values found in Array_B are not already in Array_A.

In my mind, I’m thinking I need to use 2 FOREACH loops, but then I came across array_dif() and in_array()…

Advice is appreciated… Which approach should I use?

$finalArray = array_merge($array1, array_diff($array1, $array2));