u cant shut down the backlight.. u can juz adjust it to be dimmer in the setting..
go setting>>general>>personalisation>>display>>light sensor.. adjust it to the lowest setting. Note: u will get darker screen display when u set to the lowest setting.

bro, do u actually notice that the backlight activation is based on the light sensor located next to the secondary camera?? is not ur phone backlight wont turn on or wat..the light sensor will determine when to shutdown the backlight. if u are standing under bright light then the sensor senses strong light it will turn off the backlight to save more power.. on the other hand, if u sit inside cinema which ald very dark..the backlight will turns on immediately so that u can see the keypad clearly.

however, the sensor still not very accurate and it might made some mistake so u will feel that it quite dumb at some time. what i mean was the backlight might be turn on in very bright environment.. so it depends how u use ur phone.. hope u understand wat i said..