<input type="checkbox" id="box[]" name="box[]" value="<?=$id?>" style="width: auto;">

    if (count($box)>0)
    {
        for($i=0; $i<count($box); $i++)
        {
         $boxtxt =  $boxtxt .'|'.$box[$i];
        }				  
    }

<form action="checkbox-form.php" method="post">

Which buildings do you want access to?

<input type="checkbox" name="formDoor[]" value="A" />Acorn Building

<input type="checkbox" name="formDoor[]" value="B" />Brown Hall

<input type="checkbox" name="formDoor[]" value="C" />Carnegie Complex

<input type="checkbox" name="formDoor[]" value="D" />Drake Commons

<input type="checkbox" name="formDoor[]" value="E" />Elliot House

<input type="submit" name="formSubmit" value="Submit" />

</form>

<?php
  $aDoor = $_POST['formDoor'];
  if(empty($aDoor)) 
  {
    echo("Вы не выбрали ни одного здания.");
  } 
  else
  {
    $N = count($aDoor);

    echo("Вы выбрали $N здание(й): ");
    for($i=0; $i < $N; $i++)
    {
      echo($aDoor[$i] . " ");
    }
  }
?>