First, run bitcoind on your server. Set up bitcoin.conf to have the correct RPC port and a password and etc.

Then, include the jsonRPCClient.php file:

include('jsonRPCClient.php');

Then, instantiate an object like so:

$bitcoind = new jsonRPCClient("http://$YOUR_RPC_USER:$YOUR_RPC_PASS@$YOUR_RPC_HOST:$YOUR_RPC_PORT/");

You can now run commands from the RPC using this object.

$bitcoind->getnetworkhashps();
$bitcoind->getrawtransaction($txid);

So if you want a new address:

$newaddr = $bitcoind->getnewaddress();