SOLVED

if ( isset( $_POST[\'submit\'] ) ) {
    $option_name = \'order_ref\' ;
$counter = get_option( \'order_ref\' );

//Limit the counter, based on the range of orders you will be getting per day
if ($counter <= 99 ) {
$counter = $counter + 1;
} else {
$counter = 1;
}

if ( get_option( $option_name ) !== false ) {
// The option already exists, so we just update it.
update_option( $option_name, $counter );

} else {
$deprecated = null;
$autoload = \'no\';
add_option( $option_name, $counter, $deprecated, $autoload );
}