Undefined index when submitting HTML form via POST method-Collection of common programming errors

I have been searching days why this piece of code is not working. Adjusted the code multiple times by searching for posts with the same problem.

Problem: my php file doesn’t seem to get the values it needs from a simple HTML form (made w. bootstrap) via POST-method. I use the issit function to check if the submit button has been clicked and thereafter I use to check if the form was filled out completely.

Please note that I am relatively new to php. I hope I did miss something and it is not just a simple typo.

PHP code (bootstrap/php/boeking.php)

Originally posted 2013-11-09 21:09:29.