PHP Converts the range of strings into single quotes for each matrix

I have an simple question how to add single quote ' to every array.

Example:

$a = array("1,2,3,4,5,6");

then convert it to be:

$a = array('1', '2', '3', '4', '5', '6');

Let me know your suggestion.

---

$a = explode(',', array_shift($a));