When viewing SSL enabled php.net sites, I get invalid certificate errors. Why?

The PHP project uses CAcert certificates, which are free. Most browsers and systems do not include this root certificate by default (yet), so we recommend installing it yourself. A few notes:

• Instructions for installing a root CAcert certificate for your browser: http://wiki.cacert.org/BrowserClients
• Inclusion status. The status of how/when/where the CAcert root certificate is installed by default: http://wiki.cacert.org/InclusionStatus