uses IdMultipartFormData;

{ .... }

procedure TForm1.Button1Click(Sender: TObject);

var

  data: TIdMultiPartFormDataStream;

begin

  data := TIdMultiPartFormDataStream.Create;

  try

    { add the used parameters for the script }

    data.AddFormField('param1', 'value1');

    data.AddFormField('param2', 'value2');

    data.AddFormField('param3', 'value3');

    { Call the Post method of TIdHTTP and read the result into TMemo }

    Memo1.Lines.Text := IdHTTP1.Post('http://localhost/script.php', data);

  finally

    data.Free;

  end;

end;