woocommerce_email_heading_customer_invoice │ filter-hook │ WC 1.0

Usage

add_filter( 'woocommerce_email_heading_customer_invoice', 'wp_kama_woocommerce_email_heading_customer_invoice_filter', 10, 3 );

/**
 * Function for `woocommerce_email_heading_customer_invoice` filter-hook.
 * 
 * @param  $format_string 
 * @param  $object        
 * @param  $that          
 *
 * @return 
 */
function wp_kama_woocommerce_email_heading_customer_invoice_filter( $format_string, $object, $that ){

	// filter...
	return $format_string;
}

$format_string

-

$object

-

$that

-

Where the hook is called

return apply_filters( 'woocommerce_email_heading_customer_invoice', $this->format_string( $heading ), $this->object, $this );

Where the hook is used in WooCommerce