This is why that text message turns off your iPhone and how you can fix it.

So a new bug in the iOS messages app has been discovered and people are sending it to each other to turn off/reset their phone.

The bug is replicated by sending the following string of text to another user via iMessage:

 لُلُصّبُلُلصّبُررً ॣ ॣh ॣ ॣ 冗

A lot of people on twitter seem to suspect that the Arabic text is causing the phone to reset. This, however, isn't the whole story.

How it works.

Firstly, it doesn't turn off or reset the phone, It just crashes springboard (The name of the home screen on your phone). It does this because it trys to truncate the text to fit it in the text preview window. However, Arabic text doesn't conform to English text, where removing a character always makes the text length shorter. The OS tries to shorten the text which makes it longer, this confuses the OS and causes the phone to go into a loop and fills the memory allocated to the springboard. 
Springboard realises that it needs to crash and restart to protect and fix itself (In the same way your computer will show the Blue Screen of Death, kernal panic, or your web browser will close the webpage you're viewing telling you that something went wrong).

Secondly, this phone will only restart if the person receiving the message isn't already in the Message app. This is because it's the notification that is handled by springboard and causes it to reset.

How to Fix my iPhone.

Some people lose access to the Message app after receiving this text, and there's a work around to get it back.  

You need to get those unicode characters removed from the preview pane of the message app, so that your phone doesn't try to read them when you open the app and crash again. There are a couple of ways of doing this. 

1) if you have Siri enabled, dictate a text message to the person who sent you the message. "Hey Siri, send a reply to my latest text saying 'thanks, your tried to break my phone. Aren't you a cool person'. " should do the trick. 

2) You can go to any app that allows you to share something via text or iMessage (such as the photo app) and send a photo to the person who sent you the message.

Again, the aim here is to remove the string of characters from the message preview pane. 

3) I'm gonna go out on a limb and say that the Mac Message app isn't affected by this, so you could potentially open the Message app on your Mac and delete the message there and wait for it to sync across to your phone.

How do I stop this from happening again?

To prevent this from happening again, until Apple issues a patch, you can then go to your settings and turn off text previews.