The "//ignore" option doesn't work with recent versions of the iconv library. So if you're having trouble with that option, you aren't alone.
That means you can't currently use this function to filter invalid characters. Instead it silently fails and returns an empty string (or you'll get a notice but only if you have E_NOTICE enabled).
This has been a known bug with a known solution for at least since 2009 years but no one seems to be willing to fix it (PHP must pass the -c option to iconv). It's still broken as of the latest release 5.4.3.
Here's a workaround...
$text= mb_convert_encoding($text, 'UTF-8', 'UTF-8');
That will strip invalid characters from UTF-8 strings (so that you can insert it into a database, etc.). Instead of "none" you can also use the value 32 if you want it to insert spaces in place of the invalid characters.