Problem fixed!
Ingolme, dsonesuk - thank you both for the kind assistance!
I used netBeans as my editor and only for this example, I've tried Notepad.
As I said, I have no code in the test.php file. only the 3 lines of code I wrote which are the opening php tag, closing tag and in the middle, the session_start(); function.
After googling about BOM, I've decided to install Notepad++ (version 6.9.2) which gives you the option of saving the file without the BOM.
Problem got fixed.
For someone who bumped into this thread in the future:
It seems the only problem was indeed, the BOM as Ingolme has explained.
Notepad++ gives you the option to save without BOM. (under Encoding->Encode in UTF-8 without BOM)
In addition, not just that the BOM character sent to the output, but also could cause (at least to Google Chrome) a not welcoming space at the top of the page.
First time I'm experiencing this. also the first time I'm using JetBrains for php coding.
Thanks again!