The join function will operate only on values and not keys, if you want to include keys then need to manipulate the array to have keys data in the Value itself...
What I mean:
$array = array(
'red' => 'APPLE',
'yellow' => 'BANANA',
'orange' => 'ORANGE'
);
echo (join(", ", $array));
Will output: APPLE, BANANA, ORANGE
That does not contain the key information...