Usually they stay in separate subdirectory (i.e. adodb, pdflib, WACT, smarty)
so I don't see any problem.
See:
deb http://people.debian.org/~dexter php5.1 experimental
This is my proof of concept:
# dpkg -l php\*pear
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err:uppercase=bad)
||/ Name Version Description
+++-==================-==================-================
ii php-pear 5.0.4-3 PEAR - PHP Extension and Application
ii php5.1-pear 5.0.999-5.1.0b3-0. PEAR - PHP Extension and Application
# pear config-show | grep 'PEAR directory'
PEAR directory php_dir /usr/share/php
# pear5.1 config-show | grep 'PEAR directory'
PEAR directory php_dir /usr/local/share/php
# pear5.1 list
(no packages installed)
# pear5.1 install -a DB
downloading DB-1.7.6.tar ...
Starting to download DB-1.7.6.tar (-1 bytes)
So a three installations of PEAR exists in system peacefully. The first is
official php5 Debian package, the second is my php5.1 package and the third
is package manually installed into /usr/local/share/php.