As you may have already noticed while browsing the list of existing packages, the PEAR packages provide a (often abstract) solution for a general problem. Thus your code will most likely fit into PEAR if it solves a problem that will not only occur in one (e.g. your) specific application, but that occurs in a lot of (web) applications. Examples are:
The PEAR packages that provide XML parsing functionalities can be found in the category browser.
No matter which area is covered by a package, the API should be as abstract as possible (while not becoming too complex), so that it can be utilized painlessly in as much use cases as possible.