Transliterates text from Unicode to US-ASCII.
string $string: The string to transliterate.
string $langcode: (optional) The language code of the language the string is in. Defaults to 'en' if not provided.
string $unknown_character: (optional) The character to substitute for characters in $string without transliterated equivalents. Defaults to '?'.
string $string with non-US-ASCII characters transliterated to US-ASCII characters, and unknown characters replaced with $unknown_character.
public function transliterate($string, $langcode = 'en', $unknown_character = '?');