How to generate a .po file?

FOR php:

I have a file like ---- test_1.php ----

<?php
  echo _("Test phrase");
  echo gettext("Test phrase two");
  // NOTE: _ == gettext()
?>

File -> New catalog... In the "Source paths" tab, you have to click in New Folder, Add the path of the folder in which do you have the file (in this example test_1.php)

... They caught ALL the strings that get invoked with gettext() or _() ... The useful for those function is explained here http://www.php.net/manual/es/function.gettext.php

AND... If do you use Apache (for example in Xampp), please, Stop and Start (Restart) it when do you wanna test a new change in a defined language with gettext(), in another way the change will not be affected

PoEdit is the tool to create the .po file. You have to do a little configuration with it and it's not the easiest. The way it should work is that it finds all the text in your source code that's wrapped in the marker string that you configure. This becomes the source text that you give to your translator. They then translate it into the target language(s) and save the translation files and return them to you. You then dump those files into a special directory in your php application and whenever the gettext extension sees one of the translation strings it fetches the appropriate translation.

I can probably help more if you show some details about where you got hung up with poedit.