I read on the WordPress Support forums “Error establishing a database connection on OS X 10.5” where jonokane had the novel solution of changing the database host (DB_HOST) to capital L, ‘Localhost’.

mrogers writes that “This is definitely a bug in WordPress 2.5, because every other web app I have running on this machine uses “localhost” and it connects to the MySQL server just fine.”

The problem can be isolated to the PHP function mysql_connect() and its error return value is being suppressed:

Warning: mysql_connect() [function.mysql-connect]: Can't connect to local MySQL server through socket '/var/mysql/mysql.sock'

There is quite a bit of misinformation about what the problem and solution out there. The Apple doc “Mac OS X Server 10.4.4: Improvements to Apache/PHP/MySQL interaction” from January 2006 gives clues as to the problem. On Server, a custom MySQL is included and the doc includes:

If you have downloaded and installed MySQL yourself but are using the pre-installed version of PHP, note that your custom version of MySQL might be configured to use the old MySQL socket location, /tmp/mysql.sock. The version of PHP in this software update uses the newer location /var/mysql/mysql.sock by default.

The problem with Apple using the “newer location” is that MySQL hasn’t had a major release since 2005 and so their packages don’t use the newer location, and there is no custom MySQL package for the rest of us. We have to download the regular MySQL 5 package from MySQL.com.

Jonokane’s novel solution likely works because instead of using a socket for the local connection, it attempts a TCP/IP connection, and ‘Localhost’ resolves to ‘localhost’. That will only solve the problem for WordPress though, and you would have the same problem with other PHP applications.

The correct solution is to:

$sudo cp /etc/php.ini.default /etc/php.ini
$sudo chmod +w /etc/php.ini

And in your favorite editor find the line that starts mysql.default_socket and update it to read

mysql.default_socket = /tmp/mysql.sock

Now restart Apache, and it should be golden.

You won’t encounter this problem if you are using the packages from Mac Ports or MAMP, but those would be their own adventures.

Congratulations to the Drupal community!