This will associate a given FeedItem with an AdGroup.

createCallAdGroupFeed.php

<?php

protected function createCallAdGroupFeed(AdWordsUser $user, $feed, $feed_item, $adgroup)
    {
        // Get the CampaignFeedService, which loads the required classes.
        $adGroupFeedService = $user->GetService('AdGroupFeedService');

        $feedFunctionRequestContextOperand = new RequestContextOperand();
        $feedFunctionRequestContextOperand->contextType = 'FEED_ITEM_ID';

        $feedItemFunction = new FeedFunction();
        $feedItemFunction->lhsOperand = array($feedFunctionRequestContextOperand);
        $feedItemFunction->operator = 'IN';

        $constantOperand = new ConstantOperand();
        $constantOperand->longValue = $feed_item->feedItemId;
        $constantOperand->type = 'LONG';
        $operands[] = $constantOperand;

        $feedItemFunction->rhsOperand = $operands;

        # Optional: to target to a platform, define a function and 'AND' it with the feed item ID link:
        $platformRequestContextOperand = new RequestContextOperand();
        $platformRequestContextOperand->contextType = 'DEVICE_PLATFORM';

        $platformOperand = new ConstantOperand();
        $platformOperand->type = 'STRING';
        $platformOperand->stringValue = 'Mobile';

        $platformFunction = new FeedFunction();
        $platformFunction->operator = 'EQUALS';
        $platformFunction->lhsOperand = array($platformRequestContextOperand);
        $platformFunction->rhsOperand = array($platformOperand);

        $feedItemFunctionOperand = new FunctionOperand();
        $feedItemFunctionOperand->value = $feedItemFunction;

        $platformFunctionOperand = new FunctionOperand();
        $platformFunctionOperand->value = $platformFunction;

        $combinedFunction = new FeedFunction();
        $combinedFunction->operator = 'AND';
        $combinedFunction->lhsOperand = array($feedItemFunctionOperand, $platformFunctionOperand);

        $adGroupFeed = new AdGroupFeed();
        $adGroupFeed->feedId = $feed->id;
        $adGroupFeed->adGroupId = $adgroup->id;
        $adGroupFeed->matchingFunction = $combinedFunction;
        $adGroupFeed->placeholderTypes = array(PLACEHOLDER_CALL);

        $operation = new AdGroupFeedOperation();
        $operation->operand = $adGroupFeed;
        $operation->operator = 'ADD';

        $result = $adGroupFeedService->mutate($operation);

        return $result->value[0];
    }