Phalanger should be able to directly call F # code (like any other compiled .NET code). The code will be the same as when working with standard types of PHP (although Phalanger has extensions that allow you to use .NET-specific things, such as generics - which can be difficult using PHPCOM).
The requirements for running F # code from a PHP site compiled using Phalanger are only .NET 2.0+ and IIS (to host the website). For F #, you will need to reference your F # library and FSharp.Core.dll (which contains the F # runtime and basic types).
PS: I have been working with Phalanger for some time, so if you have any questions, let me know - I can send them to the current developers (they can also provide some commercial support if you were interested).
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