I doubt you can do this simply. Thunderbird does allow scripting via the creation of plugins but I wouldn't personally know how do do it or how easy it might be.
Best answer I can think of would be to set a scheduled task / cron job to download the php to a local file then follow the instructions in the knowledge base, namely:
You can use Thunderbird to create signature files, or you can use your operating system tools to create them—for example, a plain text editor.
Thunderbird does not provide any
special place to store signature
files. You could create a Signatures
directory in your profile to store
them, making them easy to back up
along with the rest of your profile.
Or you could store them somewhere
else.
To use a signature file, specify it in
Account Settings as the signature for
an identity. Check the box "Attach the
signature from a file instead" and
specify the signature file.
This will work unless Thunderbird caches the HTML internally, however I see no indication in the FAQ that this is the case.