Minor Coding standards

Problem/Motivation

There are some files that do not comply to coding standards

Steps to reproduce

Proposed resolution

Replace $i++; and $i--; with $i += 1; and $i -= 1;
Replace all && and || in if() with a $variable
Replace all functions inside a foreach(): not foreach(array_keys($array)) but foreach($array_keys)
Add `declare(strict_types=1);` to any php file missing it

Remaining tasks

User interface changes

API changes

Data model changes