replace character ` doesn't work

Question

I have a webform where you can upload files. To replace special characters I user the following function:

function createSafeFilenameForQuestion($filename){
    $filename = str_replace(" ", "_",  $filename);
    $search  = array("ä", "ö", "ü", "ß", "Ö", "Ä", "Ü");
    $replace = array("ae","oe","ue","ss","Oe","Ae","Ue");
    $ret = str_replace($search, $replace, $filename);
    $in_charset = mb_detect_encoding($filename);
    $ret = iconv($in_charset, 'US-ASCII//TRANSLIT', $ret);
    $ret = preg_replace("/\s/i", "_", $ret);
    $ret = preg_replace("/[^0-9a-z_\.]/i", "", $ret);
    $filename = mb_strtolower($ret);
    return $filename;
}
createSafeFilenameForQuestion("ä#`´+4`32 _.png");

My problem is, that this code works fine on my local machine with PHP 5.2.5 and it doesn't work on the server with PHP 5.2.0.

local output: aeae432__.png
server output: ae

It seems, that the character ´ is the problem.

Answer 1

mbstring was installed and enabled. But as Álvarao said, using iconv($in_charset, 'US-ASCII//TRANSLIT', $ret); wasn't necessary. I removed the line and now it works.

function createSafeFilenameForQuestion($filename){
    $filename = str_replace(" ", "_",  $filename);

    $search  = array("ä", "ö", "ü", "ß", "Ö", "Ä", "Ü");
    $replace = array("ae","oe","ue","ss","Oe","Ae","Ue");
    $ret = str_replace($search, $replace, $filename);

    $ret = preg_replace("/\s/i", "_", $ret);
    $ret = preg_replace("/[^0-9a-z_\.]/i", "", $ret);
    $filename = mb_strtolower($ret);
    return $filename;
}

Input: ää#`´+4`32 _.png
Output: ae432__.png