parameter expected to be reference

Parameter 2 to Cf73rdParty_SubsAjaxCallbacks::ajax_postprocess() expected to be a reference, value given in …

Using sample codes found in Github

if(!class_exists('Cf73rdParty_SubsAjaxCallbacks')):
/**
 * Encapsulate any and all 3rd-party service callback functions
 */
    class Cf73rdParty_SubsAjaxCallbacks {
    	public function __construct(){
    		/** newsletter - mailchimp - subscribe **/

    		//actions require 2 parameters: 3rd-party response, results placeholders
    		///NOTE: customize this hook name to match your Service (in the admin settings)
            add_action('Forms3rdPartyIntegration_service_a0', array(&$this, 'ajax_postprocess'), 10, 2);
            add_action('Forms3rdPartyIntegration_service_a1', array(&$this, 'ajax_postprocess'), 10, 2);

    	}//--	function __construct
    	/**
    	 * Callback hook for 3rd-party service Mailchimp - Newsletter signup form
    	 * @param $response the remote-request response (in this case, it's a serialized string)
    	 * @param &$results the callback return results (passed by reference since function can't return a value; also must be "constructed by reference"; see plugin)
    	 */

         public function ajax_postprocess($response, &$results) {
             try {
         		// do something with $response
                
         		// set return results - text to attach to the end of the email
         		$results['attach'] = $response;

         		///add_filter('wpcf7_mail_components', (&$this, 'filter_'.__FUNCTION__));
         	} catch(Exception $ex){                
         		// indicate failure by adding errors as an array
         		$results['errors'] = array($ex->getMessage());
         	}
         }

    	// public function mailchimp_newsletter_action($response, &$results){
    	// 	try{
    	// 		// look once more for success message, in case someone didn't set up the success clause
    	// 		if( false !== strpos($response, 'please click the link in the email we just sent you') ) :
    	// 			$results['message'] = 'To complete the subscription process, please click the link in the email we just sent you.';
    	// 		else:
    	// 			$results['errors'] = array('Failed to submit response to Newsletter Signup at MailChimp');
    	// 		endif;
        //
    	// 		///add_filter('wpcf7_mail_components', (&$this, 'filter_'.__FUNCTION__));
    	// 	} catch(Exception $ex){
    	// 		$results['errors'] = array($ex->getMessage());
    	// 	}
        //
    	// 	#wp_mail( '[email protected]', 'Callback Hit', 'callback:'.__FUNCTION__."\n\ndata:\n".print_r($response,true)."\n\nresults:\n".print_r($results, true) );
    	// }//--	function mailchimp_newsletter_action

    }//---	class Cf73rdParty_MailchimpCallbacks
    //start 'em up
    $cf73rdpartycallback_instance = new Cf73rdParty_SubsAjaxCallbacks();
endif;	//class-exists

From what I could find, it seems to be some problem in conflict with php 7 above?