Revised problem 1.
Revised problem 1 and 2.
Added PHP solution to problem 6. Revised problem 1.
Revised problem 6.
Added solution to problem 20.
Added solution to problem 22.
Revised problem 22.
Added solution to problem 13.
Revised problem 22.
Merge pull request #3 from chessmasterhong/master

PHP solutions for problems 6, 13, 20, and 22.