I cannot find any information on online manual (which is IMHO missing lot's of info) is placeholder for certificate files.
Where these certificates are used? Where in my understanding there can be different certificates for different connections in example one set for HTTPS, one set for LDAP and one set for database (like MySQL) connection. So this are could be also more informative, hopefully you can improve this one in near future as well.
These parameters are here to be able to crypt (SSL) connection to MySql : between PHP server and MySql server.
It is interesting only if PHP and MySql are on different servers.