It’s been a while since I did web development on my Mac, and MySQL ceased to work. Could be related to that one time that I accidentally removed a bunch of dot-files from my home directory, could be related to MySQL upgrade. In any case, it no longer worked.
What I figured out was that the /var/mysql/mysql.sock UNIX domain socket was no longer generated. The /var/mysql folder was missing. When just creating it and assigning it to user _mysql (chown _mysql /var/mysql) did not help, I knew something was wrong with configuration.

MySQL tries to read configuration from /etc/my.cnf. To get its UNIX domain socket where PHP expects it, this needs to be contained in this configuration file:

[client]
port = 3306
socket = /var/mysql/mysql.sock


[mysqld]
port = 3306
basedir = /usr/local/mysql/
socket = /var/mysql/mysql.sock
; datadir = /servers/raiddrive/databases/

Please read the manual for details, and this is not administration advice; I’m not sure if this opens security holes, but I only use MySQL for development purposes. (Because you should know better than letting programmers near mission critical production servers.)

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via blog.vucica.net