How do I check if PHP is connected to a database already?

Have you tried mysql_ping()?

Update: From PHP 5.5 onwards, use mysqli_ping() instead.

Pings a server connection, or tries to reconnect if the connection has gone down.

if ($mysqli->ping()) {
  printf ("Our connection is ok!\n"); 
} else {
  printf ("Error: %s\n", $mysqli->error); 
}

Alternatively, a second (less reliable) approach would be:

$link = mysql_connect('localhost','username','password');
//(...)
if($link == false){
    //try to reconnect
}

mysql_ping

Try using PHP's mysql_ping function:

echo @mysql_ping() ? 'true' : 'false';

You will need to prepend the "@" to suppose the MySQL Warnings you'll get for running this function without being connected to a database.

There are other ways as well, but it depends on the code that you're using.