Sequential ASCII string generator with PHP

I was wondering how I would generate a set of ASCII strings with PHP. So the script would run and output something like:

a
b
c
d
..
x
y
z
aa
ab
ac
ad

And so on. Alpha would be good, although Alphanumeric would be much better. Also the option to define how many characters long the string can be would be a plus. IF anyone can show me how this is done, or even an idea on how I could do it, I would be very happy. All help is appreciated.

I've been thinking about it for about 2 days and just can't work out how to do it.