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PHPTAL is licensed under LGPL (v2.1 or later).
The licence does not "infect" other software you use with PHPTAL, so it's safe for commercial (closed, for-profit) use in PHP applications.
You
can redistribute
PHPTAL with applications, even if they're not free/open-source.
You only have obligations if you redistribute PHPTAL (give PHPTAL's source code to others). Sending of PHPTAL-generated HTML from a server doesn't count as redistribution.
If you
modify and redistribute PHPTAL, then your modifications of
PHPTAL's code (and only
PHPTAL's code) become
LGPL-licensed.
However, if you always keep
PHPTAL only on your server, then you're allowed to keep your modifications secret.
If you redistribute your application to others with
PHPTAL built-in, it must be possible to modify
PHPTAL part of the application. If you use copy protection (encryption/obfuscation of
PHP files), don't apply it to
PHPTAL files.
Don't remove license (
COPYING
file) from
PHPTAL copies you give to others.
IANAL. This page is only non-legally-binding summary.