php - session in login framework -

for school making login system. next step when user logged in button 'log out' wil appear on every page. know have $_session don't know how use in code. hope can me out?!

loginmodel:

function loginuser($username = null, $password = null){ $username = $_post["username"];  $password = $_post["password"];   $db = opendatabaseconnection();  $result1 = $db->prepare("select * login username = '$username' ,  password = '$password'"); $result1->execute(); if($result1->rowcount() > 0 ) {     $_session['logged in'] = true;     $_session['username'] = $username;     $db = null;     return true; }  else {     $db = null;     return false; } 

}

logincontroller:

function login(){ if(isset($_post["username"]) && isset($_post["password"])) {     if(loginuser($_post['username'], $_post['password']))     {         header("location:" . url . "login/index");     }else{             render("login/login");         echo 'ownee het een fout help';     } } else {     render("login/login"); } }