How can I modify this code ?

Question

This code was meant to be a chat system, but its works more like forum. Please can anyone help me to modify this code, so only the person who the message is from or to can see it.
Here's the code :
https://code.sololearn.com/wziPZwlowhR2/?ref=app
Here's the display part :
https://code.sololearn.com/w93TPgYMptjW/?ref=app

Answer 1

I wanted to answer this in Sololearn but Sololearn doesn't let me add PHP code in an answer for some reason.

Check my answer here:
https://justpaste.it/251t2