<?php
$step = 'quest';
$ans = array(1,2,3,4);
$questions = array($step => $ans);
var_dump($questions);

array(1) {
  ["quest"]=>
  array(4) {
    [0]=>
    int(1)
    [1]=>
    int(2)
    [2]=>
    int(3)
    [3]=>
    int(4)
  }
}