// Create ad using an existing ID.
$ad = new Ad();
$ad->id = $adId;
// Create ad group ad.
$adGroupAd = new AdGroupAd();
$adGroupAd->adGroupId = $adGroupId;
$adGroupAd->ad = $ad;
$adGroupAd->status = 'ENABLED';
// Create experiment data for experiment-only split.
$experimentData = new AdGroupAdExperimentData();
$experimentData->experimentId = $experimentId;
$experimentData->experimentDeltaStatus = 'EXPERIMENT_ONLY';
$adGroupAd->experimentData = $experimentData;
// Create operation.
$operation = new AdGroupAdOperation();
$operation->operator = 'SET';
$operation->operand = $adGroupAd;
$operations = array($operation);
// Update ad.
$result = $adGroupAdService->mutate($operations);Experiment data can only be used to include or exclude an ad from an experiment; all other properties of the ad will remain the same. If you have any questions about setting up experiments with the API you can ask us on the forum.
 - Eric Koleda, AdWords API Team
 - Eric Koleda, AdWords API Team