I had the same problem. Actually it is very simple, but I also did not find an option. He is well hidden.
Open the settings and go to:
IDE Settings β Editor β Auto Import

There you will find the checkbox "Enable auto-import in namespace". Uncheck the box and namespaces will no longer be shrunk using the use keyword.
But now PhpStorm will tell you that there is no need to use the full namespace. To avoid this, simply disable the "Unnecessary full name" check.

You can still tell PhpStorm to import the namespace of the current class. For me, he simply marks the class with the cursor and presses ALT + ENTER. Then the following dialog box appears:

When I confirm with ENTER, the namespace of this class becomes imported and shrinks: 
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