A po file example is here: http://svn.automattic.com/wordpress-i18n/pot/trunk/wordpress.pot
The Tools for edit a po file under windows is poEditor, however the website seems not correct any more.
You can check here for detail.
under Linux is http://gtranslator.sourceforge.net/ and KBabel for KDE.
Here is something for PHP
Internationalization (i18) in PHP/Quercus