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Read first: PHP functions: remove accents, UTF8 to ASCII, latinization

function UTF8_ASCII_2($str, $languages = false){
    global $tables;
    foreach($tables as $lang => $table){
        if(is_array($languages)){
            if(in_array($lang, $languages)){
                $str = strtr($str, $table);
            }    
        }else{
            $str = strtr($str, $table);
        }
    }
    return $str;
}

Usage:

$text = 'Regardez les vidéos YouTube - notamment celles proposées en Haute Définition (HD) - en toute convivialité sur grand écran.';
echo $text = UTF8_ASCII_2($text, array('latin'));

result:
Regardez les videos YouTube - notamment celles proposees en Haute Definition (HD) - en toute convivialite sur grand ecran.

$text = 'Эти продажи полностью автоматические. Имеющиеся ежечасные подписки также продолжают работать.';
echo $text = UTF8_ASCII_2($text, array('russian'));

result:
Eti prodazhi polnostyu avtomaticheskie. Imeyushiesya ezhechasnye podpiski takzhe prodolzhayut rabotat.

$text = 'Cộng hòa Xã hội chủ nghĩa Việt Nam';
echo $text = UTF8_ASCII_2($text, array('vietnamese'));

result:
Cong hoa Xa hoi chu nghia Viet Nam

Some simple table to use with strtr function:

$table1 = array(
    'Š'=>'S', 'š'=>'s', 'Đ'=>'D', 'đ'=>'d', 'Ž'=>'Z', 'ž'=>'z', 'Č'=>'C', 'č'=>'c', 'Ć'=>'C', 'ć'=>'c',
    'À'=>'A', 'Á'=>'A', 'Â'=>'A', 'Ã'=>'A', 'Ä'=>'A', 'Å'=>'A', 'Æ'=>'A', 'Ç'=>'C', 'È'=>'E', 'É'=>'E',
    'Ê'=>'E', 'Ë'=>'E', 'Ì'=>'I', 'Í'=>'I', 'Î'=>'I', 'Ï'=>'I', 'Ñ'=>'N', 'Ò'=>'O', 'Ó'=>'O', 'Ô'=>'O',
    'Õ'=>'O', 'Ö'=>'O', 'Ø'=>'O', 'Ù'=>'U', 'Ú'=>'U', 'Û'=>'U', 'Ü'=>'U', 'Ý'=>'Y', 'Þ'=>'B', 'ß'=>'Ss',
    'à'=>'a', 'á'=>'a', 'â'=>'a', 'ã'=>'a', 'ä'=>'a', 'å'=>'a', 'æ'=>'a', 'ç'=>'c', 'è'=>'e', 'é'=>'e',
    'ê'=>'e', 'ë'=>'e', 'ì'=>'i', 'í'=>'i', 'î'=>'i', 'ï'=>'i', 'ð'=>'o', 'ñ'=>'n', 'ò'=>'o', 'ó'=>'o',
    'ô'=>'o', 'õ'=>'o', 'ö'=>'o', 'ø'=>'o', 'ù'=>'u', 'ú'=>'u', 'û'=>'u', 'ý'=>'y', 'ý'=>'y', 'þ'=>'b',
    'ÿ'=>'y', 'Ŕ'=>'R', 'ŕ'=>'r',
);

$table2 = array(
    'Š'=>'S', 'š'=>'s', 'Ð'=>'D', 'Ž'=>'Z', 'ž'=>'z', 'À'=>'A', 'Á'=>'A', 'Â'=>'A', 'Ã'=>'A', 'Ä'=>'A',
    'Å'=>'A', 'Æ'=>'A', 'Ç'=>'C', 'È'=>'E', 'É'=>'E', 'Ê'=>'E', 'Ë'=>'E', 'Ì'=>'I', 'Í'=>'I', 'Î'=>'I',
    'Ï'=>'I', 'Ñ'=>'N', 'Ò'=>'O', 'Ó'=>'O', 'Ô'=>'O', 'Õ'=>'O', 'Ö'=>'O', 'Ø'=>'O', 'Ù'=>'U', 'Ú'=>'U',
    'Û'=>'U', 'Ü'=>'U', 'Ý'=>'Y', 'Þ'=>'B', 'ß'=>'Ss','à'=>'a', 'á'=>'a', 'â'=>'a', 'ã'=>'a', 'ä'=>'a',
    'å'=>'a', 'æ'=>'a', 'ç'=>'c', 'è'=>'e', 'é'=>'e', 'ê'=>'e', 'ë'=>'e', 'ì'=>'i', 'í'=>'i', 'î'=>'i',
    'ï'=>'i', 'ð'=>'o', 'ñ'=>'n', 'ò'=>'o', 'ó'=>'o', 'ô'=>'o', 'õ'=>'o', 'ö'=>'o', 'ø'=>'o', 'ù'=>'u',
    'ú'=>'u', 'û'=>'u', 'ý'=>'y', 'ý'=>'y', 'þ'=>'b', 'ÿ'=>'y', 'ƒ'=>'f'
);

$table3 = array(
            'Á'=>'A', 'À'=>'A', 'Â'=>'A', 'Ã'=>'A', 'Å'=>'A', 'Ä'=>'A', 'Æ'=>'AE', 'Ç'=>'C',
            'É'=>'E', 'È'=>'E', 'Ê'=>'E', 'Ë'=>'E', 'Í'=>'I', 'Ì'=>'I', 'Î'=>'I', 'Ï'=>'I', 'Ð'=>'D',
            'Ñ'=>'N', 'Ó'=>'O', 'Ò'=>'O', 'Ô'=>'O', 'Õ'=>'O', 'Ö'=>'O', 'Ø'=>'O',
            'Ú'=>'U', 'Ù'=>'U', 'Û'=>'U', 'Ü'=>'U', 'Ý'=>'Y',
            'á'=>'a', 'à'=>'a', 'â'=>'a', 'ã'=>'a', 'å'=>'a', 'ä'=>'a', 'æ'=>'ae', 'ç'=>'c',
            'é'=>'e', 'è'=>'e', 'ê'=>'e', 'ë'=>'e', 'í'=>'i', 'ì'=>'i', 'î'=>'i', 'ï'=>'i', 'ð'=>'d',
            'ñ'=>'n', 'ó'=>'o', 'ò'=>'o', 'ô'=>'o', 'õ'=>'o', 'ö'=>'o', 'ø'=>'o',
            'ú'=>'u', 'ù'=>'u', 'û'=>'u', 'ü'=>'u', 'ý'=>'y', 'ÿ'=>'y'
);

$table4 = array(
  'à' => 'a', 'ô' => 'o', 'ď' => 'd', 'ḟ' => 'f', 'ë' => 'e', 'š' => 's', 'ơ' => 'o',
  'ß' => 'ss', 'ă' => 'a', 'ř' => 'r', 'ț' => 't', 'ň' => 'n', 'ā' => 'a', 'ķ' => 'k',
  'ŝ' => 's', 'ỳ' => 'y', 'ņ' => 'n', 'ĺ' => 'l', 'ħ' => 'h', 'ṗ' => 'p', 'ó' => 'o',
  'ú' => 'u', 'ě' => 'e', 'é' => 'e', 'ç' => 'c', 'ẁ' => 'w', 'ċ' => 'c', 'õ' => 'o',
  'ṡ' => 's', 'ø' => 'o', 'ģ' => 'g', 'ŧ' => 't', 'ș' => 's', 'ė' => 'e', 'ĉ' => 'c',
  'ś' => 's', 'î' => 'i', 'ű' => 'u', 'ć' => 'c', 'ę' => 'e', 'ŵ' => 'w', 'ṫ' => 't',
  'ū' => 'u', 'č' => 'c', 'ö' => 'oe', 'è' => 'e', 'ŷ' => 'y', 'ą' => 'a', 'ł' => 'l',
  'ų' => 'u', 'ů' => 'u', 'ş' => 's', 'ğ' => 'g', 'ļ' => 'l', 'ƒ' => 'f', 'ž' => 'z',
  'ẃ' => 'w', 'ḃ' => 'b', 'å' => 'a', 'ì' => 'i', 'ï' => 'i', 'ḋ' => 'd', 'ť' => 't',
  'ŗ' => 'r', 'ä' => 'ae', 'í' => 'i', 'ŕ' => 'r', 'ê' => 'e', 'ü' => 'ue', 'ò' => 'o',
  'ē' => 'e', 'ñ' => 'n', 'ń' => 'n', 'ĥ' => 'h', 'ĝ' => 'g', 'đ' => 'd', 'ĵ' => 'j',
  'ÿ' => 'y', 'ũ' => 'u', 'ŭ' => 'u', 'ư' => 'u', 'ţ' => 't', 'ý' => 'y', 'ő' => 'o',
  'â' => 'a', 'ľ' => 'l', 'ẅ' => 'w', 'ż' => 'z', 'ī' => 'i', 'ã' => 'a', 'ġ' => 'g',
  'ṁ' => 'm', 'ō' => 'o', 'ĩ' => 'i', 'ù' => 'u', 'į' => 'i', 'ź' => 'z', 'á' => 'a',
  'û' => 'u', 'þ' => 'th', 'ð' => 'd', 'æ' => 'ae', 'µ' => 'u', 'ĕ' => 'e', 
  'À' => 'A', 'Ô' => 'O', 'Ď' => 'D', 'Ḟ' => 'F', 'Ë' => 'E', 'Š' => 'S', 'Ơ' => 'O',
  'Ă' => 'A', 'Ř' => 'R', 'Ț' => 'T', 'Ň' => 'N', 'Ā' => 'A', 'Ķ' => 'K',
  'Ŝ' => 'S', 'Ỳ' => 'Y', 'Ņ' => 'N', 'Ĺ' => 'L', 'Ħ' => 'H', 'Ṗ' => 'P', 'Ó' => 'O',
  'Ú' => 'U', 'Ě' => 'E', 'É' => 'E', 'Ç' => 'C', 'Ẁ' => 'W', 'Ċ' => 'C', 'Õ' => 'O',
  'Ṡ' => 'S', 'Ø' => 'O', 'Ģ' => 'G', 'Ŧ' => 'T', 'Ș' => 'S', 'Ė' => 'E', 'Ĉ' => 'C',
  'Ś' => 'S', 'Î' => 'I', 'Ű' => 'U', 'Ć' => 'C', 'Ę' => 'E', 'Ŵ' => 'W', 'Ṫ' => 'T',
  'Ū' => 'U', 'Č' => 'C', 'Ö' => 'Oe', 'È' => 'E', 'Ŷ' => 'Y', 'Ą' => 'A', 'Ł' => 'L',
  'Ų' => 'U', 'Ů' => 'U', 'Ş' => 'S', 'Ğ' => 'G', 'Ļ' => 'L', 'Ƒ' => 'F', 'Ž' => 'Z',
  'Ẃ' => 'W', 'Ḃ' => 'B', 'Å' => 'A', 'Ì' => 'I', 'Ï' => 'I', 'Ḋ' => 'D', 'Ť' => 'T',
  'Ŗ' => 'R', 'Ä' => 'Ae', 'Í' => 'I', 'Ŕ' => 'R', 'Ê' => 'E', 'Ü' => 'Ue', 'Ò' => 'O',
  'Ē' => 'E', 'Ñ' => 'N', 'Ń' => 'N', 'Ĥ' => 'H', 'Ĝ' => 'G', 'Đ' => 'D', 'Ĵ' => 'J',
  'Ÿ' => 'Y', 'Ũ' => 'U', 'Ŭ' => 'U', 'Ư' => 'U', 'Ţ' => 'T', 'Ý' => 'Y', 'Ő' => 'O',
  'Â' => 'A', 'Ľ' => 'L', 'Ẅ' => 'W', 'Ż' => 'Z', 'Ī' => 'I', 'Ã' => 'A', 'Ġ' => 'G',
  'Ṁ' => 'M', 'Ō' => 'O', 'Ĩ' => 'I', 'Ù' => 'U', 'Į' => 'I', 'Ź' => 'Z', 'Á' => 'A',
  'Û' => 'U', 'Þ' => 'Th', 'Ð' => 'D', 'Æ' => 'Ae', 'Ĕ' => 'E'
);

Some simple function to remove accent:

mb_regex_encoding('UTF-8');
mb_internal_encoding('UTF-8');

function remove_accent($str)
{
    $match = array(
        '[àâäåãáÂÄÀÅÃÁæÆ]', '[ß]', '[çÇ]', '[Ð]', '[éèêëÉÊËÈ]',
        '[ïîìíÏÎÌÍ]', '[ñÑ]', '[öôóòõÓÔÖÒÕ]', '[ùûüúÜÛÙÚ]', '[¥_Ý_ýÿ]'
    );
    $replace = array('a', 'b', 'c', 'd', 'e', 'i', 'n', 'o', 'u', 'y');
    foreach($match as $key=>$value){
        $str = mb_ereg_replace($value, $replace[$key], $str);
    }
    return $str;
}

Example:

$text = 'Regardez les vidéos YouTube - notamment celles proposées en Haute Définition (HD) - en toute convivialité sur grand écran.';
echo $text = remove_accent($text);

result:
Regardez les videos YouTube - notamment celles proposees en Haute Definition (HD) - en toute convivialite sur grand ecran.

function remove_accent_vi($str) {
    if(!$str) return false;
    $utf8 = array(
                'a'=>'á|à|ả|ã|ạ|ă|ắ|ặ|ằ|ẳ|ẵ|â|ấ|ầ|ẩ|ẫ|ậ|Á|À|Ả|Ã|Ạ|Ă|Ắ|Ặ|Ằ|Ẳ|Ẵ|Â|Ấ|Ầ|Ẩ|Ẫ|Ậ',
                'd'=>'đ|Đ',
                'e'=>'é|è|ẻ|ẽ|ẹ|ê|ế|ề|ể|ễ|ệ|É|È|Ẻ|Ẽ|Ẹ|Ê|Ế|Ề|Ể|Ễ|Ệ',
                'i'=>'í|ì|ỉ|ĩ|ị|Í|Ì|Ỉ|Ĩ|Ị',
                'o'=>'ó|ò|ỏ|õ|ọ|ô|ố|ồ|ổ|ỗ|ộ|ơ|ớ|ờ|ở|ỡ|ợ|Ó|Ò|Ỏ|Õ|Ọ|Ô|Ố|Ồ|Ổ|Ỗ|Ộ|Ơ|Ớ|Ờ|Ở|Ỡ|Ợ',
                'u'=>'ú|ù|ủ|ũ|ụ|ư|ứ|ừ|ử|ữ|ự|Ú|Ù|Ủ|Ũ|Ụ|Ư|Ứ|Ừ|Ử|Ữ|Ự',
                'y'=>'ý|ỳ|ỷ|ỹ|ỵ|Ý|Ỳ|Ỷ|Ỹ|Ỵ',
    );
    foreach($utf8 as $ascii=>$uni) $str = preg_replace("/($uni)/i",$ascii,$str);
    return $str;
}

Example:

$text = 'Cộng hòa Xã hội chủ nghĩa Việt Nam';
echo $text = remove_accent_vi($text);

result:
Cong hoa Xa hoi chu nghia Viet Nam

function remove_accent_2($str){
    $search = explode(",","ç,æ,œ,á,é,í,ó,ú,à,è,ì,ò,ù,ä,ë,ï,ö,ü,ÿ,â,ê,î,ô,û,å,e,i,ø,u");
    $replace = explode(",","c,ae,oe,a,e,i,o,u,a,e,i,o,u,a,e,i,o,u,y,a,e,i,o,u,a,e,i,o,u");
    return str_replace($search, $replace, $str);
}

Example:

$text = 'Regardez les vidéos YouTube - notamment celles proposées en Haute Définition (HD) - en toute convivialité sur grand écran.';
echo $text = remove_accent_2($text);

result:
Regardez les videos YouTube - notamment celles proposees en Haute Definition (HD) - en toute convivialite sur grand ecran.