merge 2 array key values into one key value php

lets say i have an array like this:

Array
(
    [0] => first one
    [1] => second
)

Essentially i want to get both of the values and put them into the same value but separated with a comma.

The desired output would be this:

Array
(
    [0] => first one, second
)

I'm not sure what function can achieve this

via Chebli Mohamed