shortcode to add recipient's first name to Quick Mail email message

quick-mail-first-name-shortcode.php

<?php
// quick-mail-first-name-shortcode.php 
/**
 * get first name of Quick Mail recipient.
 * for shortcode to add recipient's first name to Quick Mail email message.
 * @return string first name of recipient or email address. question mark is n/a.
 * @see https://wheredidmybraingo.com/tag/quick-mail/ Quick Mail WordPress Plugin
 * @see https://wordpress.org/support/topic/add-user-name-to-email/ support request
 */
function qm_get_first_name() {
	$first = '?';
	if ( empty( $_POST['qm-email']) ) {
		return $first;
	}

	$text = urldecode( $_POST['qm-email'] );
	$all = explode( ' ', $text );
	$j = count( $all );
	if ( 3 < $j ) {
		$j = 3;
	}
	switch ( $j ) {
		case 3:		$first = substr( $all[0], 1 );
					break;
		case 2:		$first = substr( $all[0], 1, -1 );
					break;
		default:		$first = $all[0];
					break;
	} // end switch
	return $first;
} // end qm_get_first_name
add_shortcode('first_name', 'qm_get_first_name');