//  http://php.net/manual/en/language.references.return.php

function &collector() {
  static $collection = array();
  return $collection;
}
$collection = &collector();
$collection[] = 'foo';

$collection2 = &collector();
$collection2[] = 'baz';

var_dump($collection);
var_dump($collection2);

// another usage

function &collector() {
  static $collection = array();
  return $collection;
}
array_push(collector(), 'foo');