codice:
$aMenuApp[0][parent_id]=0;
$aMenuApp[0][title]="Primo";

$aMenuApp[1][parent_id]=2;
$aMenuApp[1][title]="Secondo";

$aMenuApp[2][parent_id]=1;
$aMenuApp[2][title]="Terzo";

$aMenuApp[3][parent_id]=2;
$aMenuApp[3][title]="Quarto";

$aMainMenu = array();

foreach($aMenuApp as $key => $nodo) {
  if($nodo['parent_id'] == 2) {
    $aMainMenu[] = $nodo;
  }
}
Cosi va?

ciao