preg_replace('/'.preg_quote($my_str).'/i', '', $string);

preg_replace(): Unknown modifier ''

preg_replace('~'.preg_quote($my_str).'~i', '', $string);
              ^                       ^

$my_str = str_replace('/', '/', preg_quote($my_str));
$some_str = preg_replace('/'.$my_str.'/i', '', $string);

echo preg_replace('/'.preg_quote("$my_str",'~').'/i', '',$string );
                                            ^