connecting to non-standard mysql socket location

I had the same problem. I'm working on a hosted box (otherwise I wouldn't be using PHP :) j/k), so I can't be running around making symlinks wherever I want, now can I? This change seems to work so far:

In the file includes/database.mysql.inc change the db_connect function as follows:

function db_connect($url) {
  $url = parse_url($url);

  // Allow for non-standard MySQL port.
  if (isset($url['port'])) {
     $url['host'] = $url['host'] .':'. $url['port'];
  }

  /** Added to allow socket file in path; last path element becomes DB name *****/
  $p = explode('/', $url['path'] );
  $plen = count($p);
  if ($plen > 1) {
    $url['path'] = '/' . array_pop($p);
    $url['host'] = $url['host'] .':'. join('/', $p);
  }
  /***********************************/

  $connection = mysql_connect($url['host'], $url['user'], $url['pass'], TRUE) or die(mysql_error());
  mysql_select_db(substr($url['path'], 1)) or die('unable to select database');

  return $connection;
}

Then in your /sites/default/settings.php file you use a DB URL string like mysql://username:password@localhost:/path/to/socket/dbname. (The last element of the path is the database name).

In my case the socket is in my user home folder, so I did this:

$mysql_socket = $_ENV["HOME"] . "/mysql/socket";
$db_url = 'mysql://username:password@localhost' . $mysql_socket . '/drupal';
$db_prefix = '';

Misc. Dept.:
My hosting provider places the socket info in ~/.my.cnf but for some reason PHP doesn't pick it up... I have no idea if this is a Drupal problem or a problem w/ the way the host configured PHP...

Don't know if this is still an issue, but I found this in a search when I was trying to do the same thing before I figured it out:

$db_url = 'mysql://user:password@localhost%3Aport%3A%2Fpath%2Fto%2Fsocket%2Ffile/database';

No code changes or system changes necessary.

Why it works: parse_url returns this string as the host part:
localhost%3Aport%3A%2Fpath%2Fto%2Fsocket%2Ffile

Then it's immediately run through urldecode to get:
localhost:port:/path/to/socket/file

Which is the proper format for mysql_connect to parse.

I solved the problem without changing code or doing anything else unusual by editing the php.ini file for my server and changing

mysql.defautl_socket =

to

mysql.default_socket = /full/path/tol/mysql.sock

Then restart the apache server and just use localhost as the host and nothing in the port number.