Help needed in Internacionalization using .po files

["Francisco (São Paulo-BR)" <avs1001@xxxxxxxxxxxx>]

I already have my .po files properly translated and the folder structure 
is like following:
\locales\pt_BR.po
\locales\es_ES.po
\locales\de_DE.po

I cant find PHP material explaining how I can access the translations 
keys, etc.

Any help appreciated.