Is this really related to the PHP version? I mean, the code is trying to access an array index that doesn't exist - I would think that would cause problems in any version.

Yes it is - in the sense that if pre 7.4 a null result of array access was accepted by your code, now there's a warning.

Oh, okay. Well, I'm pretty sure that just setting a "table" value in the query string for Special:CargoExport won't work with any PHP version - although the error message is probably different, depending on the version.