How to call abstract method from interface
The normal way would be to instantiate and call non-statically like this:

(new \craft\base\Volume)-&gt;fileExists($path);

...However, that won't work here because it's an abstract class, which, by definition, cannot be instantiated. Assuming you're dealing with an Asset element(?), try calling getVolume() and then you should be able to call fileExists() from there. E.g:

$asset-&gt;getVolume()-&gt;fileExists();