Use the `callable` type

[dmitrii-fediuk]

• php.net/manual/en/language.types.callable.php
• Declare argument types for functions and methods #174 (comment)
• stackoverflow.com/questions/29730720

callable is supported by PHP ≥ 5.4:

function a(callable $f) {return $f();}
a(function() {return 'TEST';});

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