See Related Software for a list of known interfaces to the Standard Engine.
Yes. See Related Software.
Yes. Informix just released some Type IV JDBC drivers. Although not tested against a Linux platform, the type IV JDBC driver seems to work with the Linux version of SE. Since it was not formally tested, YMMV. See Related Software.
Yes, DBD::Informix. See Related Software.
The latest version of PHP (3.0.3) supports Informix. See Related Software.