<?php do {

$photoid = $row_allphotos['photoid'];

//  Multidimensional Array

$photos = array(
 "$photoid" =>array(
    '1',
    '4',
    '2'
  )
);

} while ($row_allphotos = mysql_fetch_assoc($allphotos));

$_SESSION['photos'] = $photos; 

var_dump($_SESSION);