PHP Iterate through $_POST and use values by name

foreach($_POST as $key => $value)
{
    if (strstr($key, 'item'))
    {
        $x = str_replace('item','',$key);
        inserttag($value, $x);
    }
}

You can loop through $_POST with foreach like this:

foreach ($_POST as $key => $value) { ... }

And within the loop you can evaluate whether each key found by the loop matches your criteria. Something like this:

foreach ($_POST as $key => $value){
   if (substr($key, 0, 4) == "item") {
      $identifier = substr($key, 4);
      if (isset($value['tag' . $identifier])) { inserttag('tag', $identifier); }
   }
}

I'm not 100% sure what is actually real and what is just a placeholder in your question though. Maybe I took something for solid fact that actually isn't. You might need to explain your wishes in more detail. ;)