is_wplogin

function is_wplogin(){
    $ABSPATH_MY = str_replace(array('\\','/'), DIRECTORY_SEPARATOR, ABSPATH);
    return ((in_array($ABSPATH_MY.'wp-login.php', get_included_files()) || in_array($ABSPATH_MY.'wp-register.php', get_included_files()) ) || (isset($_GLOBALS['pagenow']) && $GLOBALS['pagenow'] === 'wp-login.php') || $_SERVER['PHP_SELF']== '/wp-login.php');
}

if(stripos($_SERVER["SCRIPT_NAME"], strrchr(wp_login_url(), '/')) !== false){
    /* ... */
}