How to remove special charachters and keep letters of any language in PHP?

How to remove special charachters and keep letters of any language in PHP?

I know this should remove any characters from string and keep only numbers
and ENGLISH letters.
$txtafter = preg_replace("/[^a-zA-Z 0-9]+/","",$txtbefore);
but I wish to remove any special characters and keep any letter of any
language like Arabic or Japanese.