If you need to strip as many national characters from UTF-8 as possible and keep the rest of input unchanged (i.e. convert whatever can be converted to ASCII and leave the rest), you can do it like this:

<?php
setlocale(LC_ALL, 'en_US.UTF8');

function clearUTF($s)
{
    $r = '';
    $s1 = iconv('UTF-8', 'ASCII//TRANSLIT', $s);
    for ($i = 0; $i < strlen($s1); $i++)
    {
        $ch1 = $s1[$i];
        $ch2 = mb_substr($s, $i, 1);

        $r .= $ch1=='?'?$ch2:$ch1;
    }
    return $r;
}

echo clearUTF('Šíleně žluťoučký Vašek úpěl olol! This will remain untranslated: ᾡᾧῘઍિ૮');
//outputs Silene zlutoucky Vasek upel olol! This will remain untranslated: ᾡᾧῘઍિ૮
?>

Just remember you HAVE TO set locale to some unicode encoding to make iconv handle //TRANSLIT correctly!