Looking to sort ascending if clicked once, descending if clicked again.(PHP)?

[Spazz]

This is a sort snippet from the entire page, as of now it only sorts the first way (ASC for all except pr, which sorts DESC)

I am trying to get it to sort ascending if you click the title, then if you click it again it sorts it descending, then back to ascending if you click it again, etc...

http://pastebin.com/1NAv9M2B