A rough rule of thumb is that code which solves a problem that
exists in one or only very few projects is not convenient for
inclusion into PEAR because most people will not benefit from it.
However, this does not mean that "niche software"
cannot be included in PEAR. In the Math section we have a number of
elaborate packages that solve different kinds of algebraic
problems. While those problems do not occur in average applications,
the packages are very abstract and scientific projects benefit a
lot.
Apart from code that does not fit into PEAR because of a lack of
abstractness, there is another type of code that currently cannot be
contributed to PEAR: Full web applications.
Despite the fact that the "A" in PEAR stands for
"Application" there is a simple reason why applications
are not yet supported: Until now nobody has gotten around to make
the infrastructure like the PEAR installer capable of handling
packages that provide complete web applications.
When the situation changes, this site will be updated.