If you use PHP7 you might get a warning about deprecated class constructors. The simplest thing to do here is to rename the existing constructor to __construct() and create a new stub function with the old constructor name. The stub function points to the new constructor, so it'll work in both PHP 4, 5 and 7. See example below.
// PHP 4 Constructor
function SphinxClient () {
self::__construct();
}
// PHP 7 Constructor
function __construct () {
// Original constructor code goes here...
}