+ only works for numerical addition in PHP (oh, and arrays to an extent as well). You need to concat usint the . instead.
$var1 . ', ' . $var2 . ', ' . $var3 . ', . . .'
Or you can use a string or printformatter. I prefer formatters myself:
$data['comment'] = sprintf('%s, %s, %s', $var1, $var2, $var3);
But I'd only suggest that if you need to separate the data in some way. If you don't, just use a plain . concat instead.