There are times when I read something in bed. When the content is in portrait mode and I turn to my side, the phone gets titled to the horizontal position. Immediately the page re-orients to landscape mode. Now that is inconvenient because I cannot read sideways. On my iPad, I can lock the orientation by sliding the orientation switch to the “lock” position. But on my iPhone, it just puts the phone in vibrate mode.
The solution is simple. You can lock the phone to always be in portrait mode for applications that support both portrait and landscape modes. (Note this will not work for apps that work only in the landscape mode. Games are a good example of this kind of apps.)
To lock the phone in portrait mode, double-click the Home button to enter multi-tasking mode. Swipe the bottom pane to the right. You will now see the ‘portrait lock’. Tap it to lock the phone into portrait mode. Tap it again to release the lock. When the phone is locked in portrait, you will see a small lock on the status bar.
See this video for a quick demo.