TMX - Translation Memory eXchange for PHP
Apparently TMX is going to be the industry standard for translation.
It stores the translation data (translation sources) in xml format. This makes them easier to read by fellow humans.
There is a nice write up about it on the IBM web site.
The Horses Mouth (La Bouche de Chevaux)
It was developed and is maintained by OSCAR (Open Standards for Container/Content Allowing Re-use)
Is TMX supported in PHP?
Yes - Well, at least it can be used in the Zend Framework.