php - How can you combine two arrays?

Question

They are simplified as follows which is enough for this question. This question is based on this answer.

1

[a][b][]

and

2

[a][c]

where both arrays has one common subarray [a].

I would like to have this

[a][c][b][]

I have run the following command unsuccessfully

array1[a] + array2[a]

Answer 1

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