Currently PEAR supports the following list of licenses:
PHP License
Apache License
LGPL
BSD style
MIT License
Other licenses may be added to this list on a case-by-case basis. Please get in touch with the PEAR developers mailing list for this.
The allowed licenses were chosen to allow integration into closed source, open source as well as commercial applications. The only limitation is that the original credits must stay in the sources. For LGPL the license additionally requires that all changes to the source code must be licensed under the LGPL to anyone the source is distributed to.
For PECL extensions that are linked into PHP, the license must be compatible with the PHP license. That means you can not GPL a PECL extension or you would be violating the GPL. Note also that if you write an extension that links against a GPL'ed library you will be violating the GPL. If you need to link against a GPL'ed library, get permission from the author of the library to use the library under a compatible license.
The license of any PEAR/PECL package can be found in the head of all source code files, inside the <license> tag of the package description file (package.xml) and also on the package homepage.