// Create a prototype function (can be any callable)
$prototype = function (A $a, B $b, $c): ?string {};

// Validate that callables match the prototype
$tests = [
    $prototype, // true
    function (A $a, B $b, $c) {}, // false - return type does not match
    function ($a, $b, $c): ?string {}, // true - arguments are contravariant
    function (A $a, B $b): ?string {}, // true - extra args don't cause errors
    function (A $a, B $b, $c, $d): ?string {}, // false - Insufficient args cause an error
    function (C $a, B $b, $c): ?string {}, // true if C is a supertype of A, false otherwise
    function (SuperTypeOfA $a, B $b, $c): ?string {}, // true
    function (A $a, B $b, $c): string {}, // true - return types are covariant
];

// Create a type from a prototype
$type = CallbackType::createFromCallable($prototype);

run_tests($type, $tests);

// ...or create a type by hand for more granular control over variance rules
$type = new CallbackType(
    new ReturnType(BuiltInTypes::STRING, ReturnType::NULLABLE | ReturnType::COVARIANT),
    new ParameterType('a', A::class),
    new ParameterType('b', B::class),
    new ParameterType('c')
);

run_tests($type, $tests);

function run_tests(CallbackType $type, array $tests)
{
    foreach ($tests as $test) {
        if ($type->isSatisfiedBy($test)) {
            echo "pass\n";
        } else {
            // CallbackType implements __toString() for easy inspections
            echo CallbackType::createFromCallable($test) . " does not satisfy {$type}\n";
        }
    }
}