The License
Akaunting software is released under the GNU GPLv3 license from the Free Software Foundation. A copy of the license is also included with every copy of Akaunting, but you can also read the text of the license here. Part of this license outlines requirements for derivative works, such as apps.
May I user another license for non-PHP files?
No. For Akaunting, the GPL license covers not only PHP but everything. Derivatives of Akaunting code inherit the GNU GPLv3 license. We require that all files (PHP, JavaScript, CSS, images, etc.) be under the GPL. That way there is no confusion about what license a given file is under.
May I sell apps?
Yes. However, you must distribute all of the files under the GNU GPLv3 license.