PHP MYSQL - Difference between 127.0.0.1 and localhost

Question

I am encountering a strange problem on my imac running OS X. For my frameworks and CMS configuration files if I use host: localhost (in db settings) an error is produced and

Answer 1

localhost will cause the MySQL client to try a UNIX socket in a standard directory. If that doesn't exist or is somewhere else, you won't be able to connect. 127.0.0.1 always uses a TCP connection.

See the manual, C.5.2.2. Can't connect to [local] MySQL server.