WordPress is a Global CMS, with very minimum requirement to install:
PHP: version 5.2.4
MySQL: version 5.0
I’m sure most of us have better server configuration than the minimum. I think most hosting have at least PHP 5.4 installed. but as a freelancer, sometimes we get a client with older version of PHP in their server. And for whatever reason they cannot/do not want to upgrade to better hosting.
If you don’t have multiple PHP version in your server, you can always install multiple PHP version in your server and pollute your server with outdated script.
Or you can spent $10 and have a separate server where you can test your code for whole year.
$10 / year, that’s only $ 0.83 / month
NameCheap.com offer a super cheap hosting at $10/year with PHP version option. And you can test your plugin/theme with various PHP version ( PHP 5.2 – 5.6 ) to make sure it’s compatible with different server setup.