PHP 5.6 as a minimum
As PHP 5.5 has reach end of life in July 2016 the minimum supported version of PHP is PHP 5.6.
Option One: Ask Your Host
Your web host should be able to tell what version of PHP is running on the server.
Option Two: Find Out Yourself
Also you can find out yourself as follows:
1) Create a new file phpversion.php
2) Add a single line of code:
<?php echo phpversion(); ?>
3) Upload the file to the your web site
4) View the file in a browser at http://www.yoursite.com/phpversion.php
5) Once you have the version number you may delete the phpversion.php file from your web site.