phantm supports external API files in XML format. For example, all internal functions, classes and constants are defined using such an API file. Not only it can import those files, but it can also generate them.
To import an API file, you can use the --importAPI option, followed by the path to the XML file. See the internal specifications for example.
phantm will be able to generate a plausible API for your functons, by collecting types on all call-sites and generating possible requirements for its arguments and return values.
To generate such an API, use the --exportAPI followed by the path to the file you wish to store the API.