Well, if you're using phoenix emulator, it's quite simple. Just simple use mysql_result and all should be well. Here, I'll do it for you.
PHP:
$q = mysql_query("SELECT online_users FROM server_status LIMIT 1");
mysql_result($q, 0);
Here is how it works. Firstly, the line where it says $q = mysql_query(....); isn't really needed. It's just to make the code look more clean. You can simply do:
PHP:
mysql_result(mysql_query("SELECT online_users FROM server_status LIMIT 1") , 0);
That will have the same output as the above. Supposing you know how the mysql_query function works, and you're already connect to a database, the mysql_result functions basically fetches the result of the query. The zero basically means the first result, incase there was more then one result.