I've got an issue with spacing in strings when I do explode(); I'm sure there is an easy fix for this

The user enters a string into a form like this:
one, two, three, four, five, six
OR like this
one,two,three,four,five,six
sometimes they enter spaces after the commas, sometimes they don't. that is up to them, but it can screw up my explosion

That string is saved as $keywords. I would like to explode $keywords so I can break it apart.

If I do this:
$exploded = explode(',', $keywords);
then, that will a blank space in front of each keyword

If I do this:
$exploded = explode(', ', $keywords); //with a space after the comma
then, that wont work for people that don't put spaces between their keywords

Is there a better way to explode? or, if not, is there code that will take off the first character of a string if it is a blank white space?