Invalid SSL certificate on pear.php.net
One of the PEAR servers had a harddisk crash and got replaced by an intermediate Server, which causes the certificate to be invalid at the moment.
From the intermediate website:
The server running pear.php.net had a fatal hard disk failure and gets replaced by a new machine this week. Until the new machine is setup, this page is up to let you continue installing PEAR packages via the PEAR installer.
The reason for the invalid PEAR certificate
The current certificate is pointing to mail.cweiske.de, which does not match the domain you visited.
Chances are high that there is no risk in accepting the certificate, but if you want to be 100% sure, you should wait until the server got replaced and the original certificate got recovered.
You are still able to install PHP extensions using the installer!