puoi controllare l'estensione e richiamare le funzioni adeguate, ma prima di tutto occhio a questa nota:

Nota: Since all GIF support was removed from the GD library in version 1.6, this function is not available if you are using that version of the GD library. Support is expected to return in a version subsequent to the rerelease of GIF support in the GD library in mid 2004. For more information, see the GD Project site.

ciao