So, you mean to highlight all pieces of code that will not work with the PHP version you are writing it in, right? That can be done here:

Preferences -> Languages & Frameworks > PHP

and the same on Mac:

PhpStorm -> Preferences (⌘,) -> Languages & Frameworks > PHP

Or with newer versions of PhpStorm in Windows:

File -> Settings -> Languages & Frameworks > PHP

then select your PHP version, for example, 7.0

This is very useful when your local system runs PHP 7.0, for example, but production is running PHP 5.5.

That way PhpStorm will warn you which parts will not work in production, show proper hints during writing code according to the selected version, etc.

Do you have composer.json in these projects? Most likely yes.

Thing is: PhpStorm by default synchronizes some settings from composer.json file. It can sync your PHP Language Level as well as Source & Test roots.

Right now it looks like PHP version defined in composer.json is 5.3 or lower (5.3 is the lowest in PhpStorm) .. so PhpStorm resets your manually configured version to the provided one by composer.json. Such sync usually happens on project opening .. hence the feeling that IDE does not remember the settings.

https://blog.jetbrains.com/phpstorm/2017/07/configuring-with-composer-in-phpstorm-2017-2/

Two possible solutions:

1. Either put a proper minimal PHP version into your composer.json (if you are writing your code using PHP 5.5 features .. so what's the point stating the 5.3 as lowest supported -- such code will fail if run using 5.3).

   For example (small part of composer.json):

   "require": {
       "php": ">=5.5",
       ...
   }
   

2. Or disable such synchronisation at Settings/Preferences | Languages & Frameworks | PHP | Composer --> Synchronize IDE settings with composer.json