Sorry, but you can
Till this NEW 64 bits AND **PHP7** XAMPP, the phalcon.so and many mcrypt.so, mbstring.so, etc. have installed perfectly.
I'm using laravel with xampp for years. It's easy to start-stop, to configure without forgetting nothing, tec.
Without been a light in Phalcon, the phalcon extension itself compiles perfectly to the local variety of computer by downloading a C source
Somewhere is a bug. For sure in the "kind" of PHP7 I use..
Perhaps I need to compile "my own" and "local" PHP7 to understand what happens ? As Phalcon does for it's extension.
It's also true, you're also right, the Ubuntu distribution updates Appache2, Php.xx etc and not xampp !!
Both of them coexist : think to xampp as a mayonnaise which mixes the salad of the ubuntu's distribution..(ha.. ha..) : xampp uses the "local" ubuntu's php5 and other ingrédients.
The trick is to start httpd as a service, **for only one of them**, the xampp's one.
For the moment, today Wed Apr 20 2016, perhaps no one installed PHP7+XAMPP7+phalcon.so together .. incredible..