Debug

Install Xdebug php

sudo apt install php-xdebug

Configure Xdebug

Example:
sudo gedit /etc/php/5.6/mods-available/xdebug.ini

Create a Drupal 8 module is a good idea, but develop as Drupal is BEST.

The famous tools PhpCS and PhpCBF allow you to check and correct (some) drupal coding standards and best practice errors.

Initialisation - Create link to the script (Optional).

ln -s core/scripts/run-tests.sh ~/bin

Run Test and open rest in web browser.

php core/scripts/run-tests.sh --browser --class "Drupal\YOUR_MODULE\Tests\JustTest"

Step 1. Install Xdebug
If not already ainstalled, Install Xdebug (I'ts already on the package WAML, LAMP, MAMP)
In Linux Debian/Ubuntu/Mint
# sudo apt-get install php5-xdebug

Step 1.
Check apache error log (use tail to show in real time)
# tail -f /var/log/apache2/error.log

Step 2 : 
Check MySQL database connection using mysql -uusername -ppassword