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I don't think including a php file with SSI will work. If you have SSI enabled for a file extension then, I don't think, adding php parsed as well will work.
I would use the php extension on that particular page and then include the file using the php include or require function.
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On Apache you may include the results of a CGI script with PHP´s virtual [php.net] function which performs a subrequest. This is similar to <!--#include virtual --> in SSI.