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 collections with a single recipient (as multiple recipients are not possible in the Reply-To header).

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

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