Using the If conditional with sessions in PostgreSQL+PHP

My doubt is quite simple probably, but as a first time for me, this things get confusing My PHP code is basically a session based one where each person gets texts to display on their page once logged in, but each person gets different ones, and those are saved on a database with their names too So I need to know how would I make the query on php to get the information belonging to my current user Something like

if(user=="John"){
  display(documents of John);
}

Of course "documents of John" is a field instead But, hopefully you can get the idea

Thanks