title: PHP 7 return type hint: Is it downward compatible?
body: <p>I am using PHP 7.1.1 to run WordPress. Now, whenever I add functions in the code of my theme or plugins, the IDE tells me to add a return type hint like this:</p>
  
  <p><a href="https://i.stack.imgur.com/kRb82.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kRb82.jpg" alt="Example"></a></p>
  
  <p>I do know how to add it, but I am unsure whether or not return type hints are compatible with older PHP version? Will I run into any problems when using them and, for instance, put the website on a server running PHP 5.x?</p>