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<?php function foo(): ?callable { return function() {}; } function bar(): ?callable { return 'strpos'; } function baz(): ?callable { return 'baz'; } function qux(): ?callable { return array(new SplFixedArray(), 'fromArray'); } function quux(): ?callable { return array('SplFixedArray', 'fromArray'); } function corge(): ?callable { return 'SplFixedArray::fromArray'; } function grault(): ?callable { return null; } foo(); bar(); baz(); qux(); quux(); corge(); grault();