• Install swig from official site or via package manager. e.g.

  # Debian based distributions
  apt install swig

• Install GTK library. e.g.

  # Debian based distributions
  apt install libgtk-3-dev

• Install PHP development package. e.g.

  # Debian based distributions
  apt install php7.4-dev

• Execute the following commands

  swig -php Source.i
  gcc `php-config --includes` `pkg-config --cflags gtk+-3.0` -fpic -c Source_wrap.c Source.c `pkg-config --libs gtk+-3.0`
  gcc `pkg-config --cflags gtk+-3.0` --shared Source_wrap.o Source.o -o <filename>.so `pkg-config --libs gtk+-3.0`

• Move the generated file to the php-config installation directory

  mv <filename>.so $(php-config --extension-dir)/

• Copy Source.php to PHP-Desktop/util/

Well, in theory it should work on windows after

• Installing swig on windows
• Installing windows version of the gtk library
• Compiling this library to a dll. You might have to download php source code and manually add/edit/remove references to libraries. Goodluck trying to get this to work.
• Move the dll to <PHP_INSTALLATION_PATH>/ext/