<?

printf( "%.2f\n", calculate_vat( 100, 25       ) );
printf( "%.2f\n", calculate_vat( 125, 25, true ) );
printf( "%.2f\n", calculate_vat( 350,  6       ) );
printf( "%.2f\n", calculate_vat( 371,  6, true ) );

function calculate_vat( $amount, $rate = 25, $incl = false ) {

    return( $amount *
      (
        ( $incl == true )
          ? ( 100 / ( $rate + 100 ) )
          : (   1 + ( $rate / 100 ) )
      )
    );
}

?>