how can i hide my photos in iphone or how can i lock my photos

You cannot really keep the photos secret on your iPhone.

Photos can be hidden from the Moments and Collections, but will always be visible in the Hidden Album or the All Photos album.

See this Help page: http://help.apple.com/iphone/9/#/iphf14943e

Hide photos or videos you want to keep but not show. While viewing thumbnails, tap Select, then select the photos or videos you want to hide. Tap , then tap Hide. The photos or videos are moved to the Hidden album.

Note: Photos or videos are hidden from Moments, Collections, and Years, but are still visible in Albums.

The only save way to prevent access to your photos is to keep the passcode to your iPhone secret.