I suspect that the field which stores the numbers is actually a varchar and that is why the sort is based upon alpha rules, not numeric rules.
You can
change the underlying table
select the value of the field and sort by the new field
* sort once you are back in php
the following link points to a javascript sorting example but wouldn't be hard to convert:
www.npac.syr.edu/projects/tutorials/JavaScript/examples/advanced/
there are some other excellent versions out there so go hunting.