Variation of pattern Inerpreter in PHP

<?php

//-----------------------------------------------------------------------------------------

function Interpret($name, &$context, $chain)
{
  if (!array_key_exists($name, $context)) $context[$name] = array();
  $con = $context[$name];
  if (!array_key_exists("start", $con)) $con["start"] = 0;

  $start = $con["start"];
  $res = true;
  for($i = $start; $i<count($chain); $i++)
  {
    if (!$chain[$i]($context))
    {
      $con["start"] = $i;
      $res = false;
      break;
    }
  }
  $context[$name] = $con;
  return $res;
}


//-----------------------------------------------------------------------------------------

session_start();

if (!array_key_exists("context", $_SESSION))
{
  $_SESSION["context"] = array();
  $_SESSION["context"]["required_values"] = array();
  $_SESSION["context"]["entered_values"] = array();
  $_SESSION["names"] = 0;
}

$_SESSION["context"]["required_values"] = array();
$_SESSION["context"]["entered_values"] = $_GET;

$prog = array();


//-----------------------------------------------------------------------------------------

$prog[] = function(&$context)
{
  $context["qwe"] = 5;
  return true;
};
$prog[] = function(&$context)
{
  $context["qwe"] *= 2;
  return true;
};
$prog[] = function(&$context)
{
  $prog = array();

  if (!array_key_exists("qwe10", $context)) $context["qwe10"] = ($context["qwe"] > 10);

  if ($context["qwe10"])
  $prog[] = function(&$context)
  {
    $context["qwe"] /= 4;
    return true;
  };
  else
  $prog[] = function(&$context)
  {
    if (array_key_exists("increment", $context["entered_values"]))
    {
      $context["qwe"] += is_numeric($context["entered_values"]["increment"]) ? $context["entered_values"]["increment"] : 0;
      return true;
    }
    else
    {
      $context["required_values"][] = "increment";
      return false;
    }
  };

  return Interpret("qwe10node", $_SESSION["context"], $prog);
};

//-----------------------------------------------------------------------------------------

$res = Interpret("main", $_SESSION["context"], $prog);

echo "Full state: ".json_encode($_SESSION["context"]);
echo "<br />";

if ($res)
{
  echo "Result: ".$_SESSION["context"]["qwe"];
  echo "<br />";
  echo "Done";
  $_SESSION = array();
}
else
{
  if (count($_SESSION["context"]["required_values"]) > 0)
  {
    echo '<form action="main.php">';

    foreach ($_SESSION["context"]["required_values"] as $name)
    {
      echo "Enter $name: <input type='text' name='$name'><br>";
    }
    echo '<input type="submit"><br></form><br>';
  }
}

//-----------------------------------------------------------------------------------------

?>