function translit_name($name)
{
$symbs = array
(
"а" => "a","б" => "b","в" => "v","г" => "g","д" => "d",
              "е" => "e","ё" => "e","ж" => "j","з" => "z","и" => "i",
              "й" => "iy","к" => "k","л" => "l","м" => "m","н" => "n",
              "о" => "o","п" => "p","р" => "r","с" => "s","т" => "t",
              "у" => "u","ф" => "f","х" => "h","ц" => "ts","ч" => "ch",
              "ш" => "sh","щ" => "sch","ъ" => "","ы" => "y","ь" => "",
              "э" => "e","ю" => "yu","я" => "ya", " " => "_");

return strtr(strtolower($name), $symbs);
}
$extensions = array(".gif", ".jpg");
$img_banner = strtolower(translit_name($_FILES['file_banner']['name']));
                $str_img_banner = strstr($img_banner, ".");

if(!in_array($str_img_banner, $extensions) && !empty($img_banner)){ .... }