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:
Support for Network protocols
Object oriented wrappers that provide easy access to otherwise complicated or less intuitive PHP extensions.
Parsing different XML dialects.
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.