Prova cosi...
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<? 
$istanza   = "pippo";
$nomeArray = "array$istanza";
${$nomeArray}[] = "uno";
${$nomeArray}[] = "due";
${$nomeArray}[] = "tre";
${$nomeArray}[] = "quattro";
${$nomeArray}[] = "cinque";
print_r (${$nomeArray});
?>


<? 
$arrayBack [] = ${$nomeArray};
print_r ($arrayBack);

 ?>
</body>
</html>