Arch Linux

This was fixed upstream. It also seems to work fine here. I guess we need more information here about your my.cnf and php.ini. The default socket in my.cnf should be /var/run/mysqld/mysqld.sock while the socket in php.ini can be left unset.

works for me too

my.cnf:
[mysqld]
socket = /var/run/mysqld/mysqld.sock

php.ini (via 'php -i'):
mysql.default_socket => /var/run/mysqld/mysqld.sock
mysqli.default_socket => /var/run/mysqld/mysqld.sock
pdo_mysql.default_socket => /var/run/mysqld/mysqld.sock

php values are empty in php.ini

Looks fine to me.

MySQL socket works fine from command line.

$ mysql --socket=/var/run/mysqld/mysqld.sock --protocol=socket -uXX -pXX test
Welcome to the MySQL monitor.
$ mysql -hlocalhost -uXX -pXX test
Welcome to the MySQL monitor.

This might actually be a PHP bug. Gotta go through PHP changelog and see what's changed.

...and a "misconfiguration" by me, since I'm connecting using "dbhost" instead of "localhost".
"dbhost" used to be another server, but due to hardware malfunction it's running locally temporarily.

For some odd reason it's using TCP instead of sockets. Problem solved by opening 3306 from iptables.

$ sudo pacman -U php-*5.3.2*xz
[...]
$ php -r "mysql_connect('dbhost', 'XX', 'XX');"
PHP Warning: mysql_connect(): [2002] Connection refused (trying to connect via tcp://dbhost:3306) in Command line code on line 1
$ sudo pacman -U php-*5.3.1-6*gz
[...]
$ php -r "mysql_connect('dbhost', 'XX', 'XX');"
$

If you define a hostname you tell mysql to use tcp. However, php will try to connect via socket if you set it to localhost. This unintuitive behavior might have confused you here. See http://de3.php.net/mysql_connect

and especially this note:

Note: Whenever you specify "localhost" or "localhost:port" as server, the MySQL client library will override this and try to connect to a local socket (named pipe on Windows). If you want to use TCP/IP, use "127.0.0.1" instead of "localhost". If the MySQL client library tries to connect to the wrong local socket, you should set the correct path as Runtime Configuration in your PHP configuration and leave the server field blank.

Maybe something to do with these:
http://bugs.archlinux.org/task/17179
http://bugs.php.net/50231

Thanks for listening :) This bug can now be closed as user error :)

Yeah, I understand TCP vs. socket difference, but I'm just confused why it was working before.

Might have been a side effect of the "self made" patch we had applied to 5.3.1.