Clean a string for use as filename by simply replacing all unwanted characters with underscore (ASCII converts to 7bit). It removes slightly more chars than necessary. Hope its useful. 

$fileName = 'Test:!"$%&/()=ÖÄÜöäü<<';
echo strtr(mb_convert_encoding($fileName,'ASCII'), 
    ' ,;:?*#!§$%&/(){}<>=`´|\\\'"', 
    '____________________________');