hmm, thanks for first comment but I´m looking for a bit more comfortable solution.
I embedded an iframe with 1px width and 1px height which opens the file "online.php" in this file has all content of "index.php" plus meta tag refresh of itself all 4 minutes and the online status is shown for 5 minutes.
Well, it´s not the best solution but till someone posts a better solution I will use this one.