Why three different versions of PHP?

I have searched before asking this question - I am surprised it has not been asked more often.

Why have the PHP teams got three different stable versions of PHP on the go?

5.6, 5.5, 5.4

And they've just recently released version 7 alpha

Could someone enlighten me as to 1) Why the PHP group decided that three different stable versions of PHP is good idea? And might I assume that I best just jump straight into 5.7 and clean up my code?

I don't think my requirements are exotic - I don't crunch data, I just use PHP validated data to read/write to MySQL - no rocket science.

The issue?

My old WAMP Zend v6 Community Edition run's PHP 5.5.7 and my new AWS micro machine uses 5.3.29 (build date May 2015 but amazingly, AWS have standardized on the pre-historic 5.3). I discovered a bug with json_encode. When I realised I have two different versions of PHP, I'm thinking its best I just upgrade both to similar versions. Hence I am thinking 5.7 is probably my best bet for future support. Comments welcome.