PHP | join() an array with of single-itemed arrays

Question

I have an array that looks like this:

array(
    array('item 1'),
    array('item 2'),
    array('item 3'),
    array('item 4')
)

I want to use the join() function in PHP to create the following string:

item 1, item 2, item 3, item 4

Obviously, join($items, ', ') doesn't work (array to string conversation)

Is there a better way doing this besides looping through the array?