Add Reply-To Email Header to Approval Email

The following snippet will add a Reply-To header to the email sent to the photographer after a collection has been approved. Please note that this only works for single collections, not multi collections with more than one recipient.

function my_picu_email_headers( $headers, $mail_context, $collection_id ) {
    if ( $mail_context == 'photographer_collection_approved' AND ! picu_is_multi_collection( $collection_id ) ) {
        $email = sanitize_email( get_post_meta( $collection_id, '_picu_collection_email_address', true ) );
        $headers .= "Reply-To: " . $email . "\r\n";
    }
    
    return $headers;
}

add_filter( 'picu_email_headers', 'my_picu_email_headers', 10, 3 );