Just do the authentication one time, approve your app, get your authentication key, and in the code:
$dbxClient = new dbx\Client($accessToken, "PHP-Example/1.0");
$accountInfo = $dbxClient->getAccountInfo();
print_r($accountInfo);
Change:
$accessToken to "{Your auth key}"
or put the line:
$accessToken = "{authentication code here}";
above that code.