Download a MailChimp contact list to CSV

add_action( 'admin_init', function() {

	// Settings
	$api_key = 'API_KEY_HERE';
	$list_id = 'LIST_ID_HERE';

	// Set Variables
	$dc = substr( $api_key, strpos( $api_key, '-' ) + 1 );
	$url = 'https://' . $dc . '.api.mailchimp.com/3.0/lists/' . $list_id;
	$args = array(
		'headers' => array(
			'Authorization' => 'Basic ' . base64_encode( 'user:'. $api_key )
		)
	);

	// Get List Information
	$response = wp_remote_get(
		'https://' . $dc . '.api.mailchimp.com/3.0/lists/' . $list_id,
		$args
	);
	$body = json_decode( $response['body'] );
	if( $response['response']['code'] != 200 ) { return; }

	// Get List Members In Batches Of Fifty
	$member_count = $body->stats->member_count;
	$rows = [];
	for( $offset = 0; $offset < $member_count; $offset += 50 ) {
		$response = wp_remote_get(
			'https://' . $dc . '.api.mailchimp.com/3.0/lists/'
				. $list_id . '/members?offset=' . $offset . '&count=50',
			$args
		);
		$body = json_decode( $response['body'] );
		if( $response['response']['code'] == 200 ) {
			foreach( $body->members as $member ) {
				$rows[] = [
					$member->email_address,
					$member->merge_fields->FNAME,
					$member->merge_fields->LNAME,
				];
			}
		}
	}

	// Output As Downloadable CSV
	header( 'Content-Type: text/csv; charset=utf-8' );
	header( 'Content-Disposition: attachment; filename=download.csv' );
	$out = fopen( 'php://output', 'w' );
	foreach( $rows as $row ) {
		fputcsv( $out, $row );
	}
	fclose( $out );
	exit;
} );