Interfaces for functions.

example.php

<?php

require __DIR__.'/function_interfaces.php';

interface foo {}

interface interfaces {
    function my_function();
    function my_function2(callable $callable, $string, \Traversable $traversable, foo $foo, $constant = DIRECTORY_SEPARATOR);
}

// True
var_dump(function_interfaces\function_implements(function () {
}, 'interfaces::my_function'));

// True - Empty interfaces are _always_ implemented
var_dump(function_interfaces\function_implements(function ($param) {
}, 'interfaces::my_function'));

// False
var_dump(function_interfaces\function_implements(function () {
}, 'interfaces::my_function2'));

// True
var_dump(function_interfaces\function_implements(function (callable $callable, $string, \Traversable $traversable, foo $foo, $constant = DIRECTORY_SEPARATOR) {
}, 'interfaces::my_function2'));

// True - Parameter names don't matter
var_dump(function_interfaces\function_implements(function (callable $callable2, $string2, \Traversable $traversable2, foo $foo2, $constant2 = DIRECTORY_SEPARATOR) {
}, 'interfaces::my_function2'));

function_interfaces.php

<?php

namespace function_interfaces;

// Inspired by https://github.com/ircmaxell/Protocol-Lib
function function_implements($function, $interface)
{
    $interfaceReflectionMethod = new \ReflectionMethod($interface);
    $functionParameters = (new \ReflectionFunction($function))->getParameters();

    $checkParam = function($param1, $param2) {
        foreach (array(
            'isArray',
            'isCallable',
            'isDefaultValueAvailable',
            'isDefaultValueConstant',
            'isOptional',
            'isPassedByReference',
            'canBePassedByValue',
            'getDefaultValue',
            'getDefaultValueConstantName',
            'allowsNull',
            'getClass',
            ) as $check) {

            try {
                // Do loose comparision, otherwise ReflectionParameter::getClass
                // check will return false
                if ($param1->$check() != $param2->$check()) {
                    return false;
                }
            } catch (\ReflectionException $e) {
                // Some methods throw exceptions when a parameter is typehinted
                // with a internal class, array or callable.
                //  * ReflectionParameter::getDefaultValue
                //  * ReflectionParameter::getDefaultValueConstantName
                //  * ReflectionParameter::isDefaultValueConstant
                // See: http://www.php.net/manual/en/reflectionparameter.getdefaultvalue.php#refsect1-reflectionparameter.getdefaultvalue-notes
                if ('Internal error: Failed to retrieve the default value' !== $e->getMessage()) {
                    return false;
                }
            }
        }

        return true;
    };

    foreach ($interfaceReflectionMethod->getParameters() as $key => $param) {
        if (!isset($functionParameters[$key])) {
            return false;
        }

        if (!$checkParam($param, $functionParameters[$key])) {
            return false;
        }
    }

    return true;
}