hello my iphone keeps switching off and then showed the apple sign . it wont turn on again unless i plugged it to the charger .

It sounds as if you may need a battery replacement and the Genius Bar is the place to have that checked out or to determine if there is some other problem and to discuss the options that best meet your needs.

Bring the charger with you when you go as well as the phone.

Make an appointment at the Genius Bar for a hardware evaluation.