If using standard strings, gettext correctly uses translated string.
If using formatted strings (those that can be used for printf, like "my name is %s and I'm %d years old"), it leaves string in original language.

Short example:

--- from test.php ---
print (_("how are you?"));
print "<br>";
printf (_("and now, when asking %d nd time?"), 2);

--- from i18n.po ---
#: test.php:6
msgid "how are you?"
msgstr "kak si kaj?"

#: test.php:8
#, c-format
msgid "and now, when asking %d nd time?"
msgstr "in sedaj, ko sprasujem %d .?"


--- output from this script ---
kak si kaj?
and now, when asking 2 nd time?


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I'm using PHP4.2.3 as apache1.3.26 module on WinXP.

Regards,
  Bostjan