WooCommerce Email Subject Line Generator

<?php
/**
 * WooCommerce email subjects and headings.
 *
 * The filter name carries the email ID, so a typo there is a filter that
 * never runs rather than an error.
 */

/**
 * Subject for the customer_processing_order email.
 *
 * @param string   $subject Current subject.
 * @param WC_Order $object  The order, or the user for account emails.
 * @param WC_Email $email   The email object.
 * @return string
 */
function my_store_customer_processing_order_subject( $subject, $object, $email ) {
	$text = __( 'Thanks {customer_first_name}, order #{order_number} is on its way', 'my-store' );

	// format_string() is what turns {order_number} into the number.
	return $email->format_string( $text );
}
add_filter( 'woocommerce_email_subject_customer_processing_order', 'my_store_customer_processing_order_subject', 10, 3 );

/**
 * Heading for the customer_processing_order email.
 *
 * @param string   $heading Current heading.
 * @param WC_Order $object  The order, or the user for account emails.
 * @param WC_Email $email   The email object.
 * @return string
 */
function my_store_customer_processing_order_heading( $heading, $object, $email ) {
	$text = __( 'Order received', 'my-store' );

	return $email->format_string( $text );
}
add_filter( 'woocommerce_email_heading_customer_processing_order', 'my_store_customer_processing_order_heading', 10, 3 );