How to write realpath function

function myrealpath($path, $check_file_exsits = true) {
    $_DS = DIRECTORY_SEPARATOR;
    $path = str_replace(
        array("/", "\\"),
        array($_DS, $_DS),
        $path
    );
    $cwd = getcwd();
    $arr = explode($_DS, $cwd);
    $len = count($arr);
    
    $arrAdd = explode($_DS, $path);
    if (isset($arrAdd[0])) {
        if ($_DS == '/') {
            if (strpos($arrAdd[0], '/') !== false) {
                $arr = array();
                $len = 0;
            }
        } else {
            if (strpos($arrAdd[0], ':') !== false) {
                $arr = array();
                $len = 0;
            }
        }
    }
    
    foreach($arrAdd as $part) {
        if (strpos($part, '..') !== false) {
            if ($len - 1 == 0) continue;
            unset($arr[--$len]);
        } elseif ($part == '.') {
            
        } elseif (!empty($part)) {
            $arr[$len++] = $part;
        }
    }
    
    $full = implode($_DS, $arr);
    if (file_exists($full) && $check_file_exsits) return $full;
    elseif (!$check_file_exsits) return $full;
    else return "";
}