Not sure exactly what you mean... What you need to do, I assume, is read in the external PHP file, e.g.:
$filename = "something.php";
$fd = fopen ($filename, "r");
$contents = fread ($fd, filesize ($filename));
fclose ($fd);
And the 'parse' it, with:
eval($contents);
Which will 'run' the PHP code, and set any variables as per the code, etc.