Sadly, due to the nature of FusionBB it's not possible to execute PHP from within a post or insert. You can execute PHP from within particle boxes which can appear on your portal or in your sidebar, so placing something like
in the code section of a particle box would allow it to execute.
You might be able to get this to work by modifing scripts, would just depend on exactly how you needed it to appear. For example, appearing on all forum posts, would be pretty easy, because you could execute the code in the script and assign the output to a variable used in the template, however it gets kinda hard if it only needed to appear in a certain post or article.
The only other way I can think to get around it, would be to include the content via an iFrame, then you're essentially displaying another page (where you can execute your script).
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