how can I hide this error!?
You really should use mysqli_ or else one day in the future when you upgrade PHP, your code will just stop working, and you'll have PHP complaining that mysqlconnect() doesn't exist. When functions are deprecated, it is a sign that you must begin migrating to the alternatives. I highly recommend using mysqli as the syntax is pretty much the same as mysql_*.
If you still insist to use mysql_*, then you can tell PHP not to report the use of deprecated functions. Look through
/etc/php/php.ini and search for
& ~E_DEPRECATED to it. For example, my PHP config has:
error_reporting = E_ALL
so I'd end up with
error_reporting = E_ALL & ~E_DEPRECATED
Hope this helps