var eru = [
   [" 12 July-20 July 2015",18]
];

$intervallo= " 10 July-16 July 2015"
$numvulc= 25;
$aggiornamento= array($intervallo,$numvulc);

$fileeru= "out/eru.js";
$json = json_decode(file_get_contents($fileeru), true);

// how many arrays there are into multidimensional array **eru**
$counteru= count($eru);

//maybe here there's problem

if($eru[$counteru] != $aggiornamento){
  $eruzioni[]= $aggiornamento;
}

$eru= json_encode($eru);
$eru= 'var eru= '.$eru.';';
file_put_contents($fileeru,$eru);

var eruzioni = [
  [" 12 July-20 July 2015",18],
  [" 10 July-16 July 2015",25]
];

var eruzioni = [
  [" 10 July-16 July 2015",25]
];