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lammert - 11:54 am on Apr 24, 2007 (gmt 0)
The hosting company could use a reverse proxy server setup for security, performance or multiplexing reasons. If the connection between your php and mysql server takes too long, the front-end proxy might decide that the hosting server is not reachable and return the 504.
The 504 is generated by a proxy server in between the visitor and your webserver. Could it be that your hosting provider has a front-end proxy server using reverse proxy to internal hosting servers? I have a reverse proxy setup to another server and it sometimes gives this error when there are problems reaching the site server.