Only process $_POST elements that begin with a certain string
$_POST['city_1']
$_POST['city_2']
$_POST['city_3']
$_POST['city_4']

<input type="text" name="cities[city_1]">
<input type="text" name="cities[city_2]">
<input type="text" name="cities[city_3]">
<input type="text" name="cities[city_4]">

foreach($_POST['cities'] as $city)
{
  echo $city;
}

$cities = preg_grep('/^city_d+$/', array_keys($_POST));

foreach($cities as $city) {
    echo $_POST[$city];
}

foreach($_POST as $name=>$value) {
    if (strpos($value, 'city_') !== 0) continue;

    echo $value;
}

foreach($_POST as $key => $value)
    if(preg_match("/^city_d+$/", $key))
        ...

function startsWith($haystack, $needle)
{
  $length = strlen($needle);
  return (substr($haystack, 0, $length) === $needle);
}


foreach ($_POST as $k=>$v)
{
  if (startsWith($k, 'city_')
  {
    // Process parameter here ...
  }
}

foreach($_POST as $key=>$value) {
  //filter based on $key
}