Pear is a very useful and popular library for PHP. Its widely Used. It has many libraries.

You can see all Pear packages by going here http://pear.php.net/packages.php

Packages for Caching and Quick Form are very popular.
It also has libraries for webservices like SOAP

In new versions of PHP, Pear is already integrated. Go here for instructions on how to install Pear.

Hope this might help somebody.

Also feel free to list information about more libraries.

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