String in php session

Use a .(dot) to concat string and variable and remove _ from $_SESSION_ try

$_SESSION[$questioncounter.'question'] = $accepted;

so full code :-

<?php
session_start();
$questioncounter = 2;
$accepted = 'yes';
$_SESSION[$questioncounter.'question'] = $accepted;
echo $_SESSION['2question'];  // yes
?>

Well, sometimes concatenation is needed but not in this case. Building variables by concatenating strings in this case would be the worst approach.

This is the way you should do things

// start session
if(!isset($_SESSION)){
    session_start();
}

// now, add your questions to the $_SESSION this way
$question = array('Is the sky really blue?', 'Should I stay or should I go?', 'Why I can\'t fly');
$_SESSION['question'] = $question;

// or this way
$_SESSION['question'][0] = 'Is the sky really blue?';
$_SESSION['question'][1] = 'Should I stay or should I go?';
...

// add their status this way
$_SESSION['question'][0]['accepted'] = 1; // or $_SESSION['question'][0]['status'] = 1;
$_SESSION['question'][1]['accepted'] = 0;

// and finally use them like this
echo $_SESSION['question'][0];

// or
if($_SESSION['question'][0]['accepted']){
    // say 'Bravo!'
}