mypmpro-prefix-mc-merge-tags-for-multiple-sites.php

<?php //do not copy

/**
 * This recipe will prefix the Mailchimp merge tags that get sent through. 
 * Each site that is connected to the same MC account that might have the same subscribers should then have this recipe on it. 
 * You can choose between a hardcoded prefix or one that takes the first 3 letters of the site name.
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

/**
 * Adds the prefix to the default fields that we add for MC
 */
function mypmpro_subscribe_merge_fields( $fields, $list_id ) {

	//Use a prefix that you prefer
	// $site_prefix = 'SWS_';

	//Generate one dynamically based on site name
	$site_prefix = strtoupper( substr( get_bloginfo(), 0, 3 ) ).'_';

	$fields = array_combine(
	    array_map( function($k){ return $site_prefix.$k; }, array_keys( $fields ) ),
	    $fields
	);

	return $fields;

}
add_filter( 'pmpro_mailchimp_merge_fields', 'mypmpro_subscribe_merge_fields', 10, 2 );

/**
 * Adds the prefix for any custom merge tags that might have been added
 * @param  [type] $fields [description]
 * @param  [type] $user   [description]
 * @param  [type] $list   [description]
 * @return [type]         [description]
 */
function mypmpro_subscribe_fields_prefix( $fields, $user, $list ) {

	//Use a prefix that you prefer
	// $site_prefix = 'SWS_';

	//Generate one dynamically based on site name
	$site_prefix = strtoupper( substr( get_bloginfo(), 0, 3 ) ).'_';

	$fields = array_combine(
	    array_map( function($k){ return $site_prefix.$k; }, array_keys( $fields ) ),
	    $fields
	);

	return $fields;

}
add_filter( 'pmpro_mailchimp_listsubscribe_fields', 'mypmpro_subscribe_fields_prefix', 99, 3 );