The PHP interfaces' cheatsheet

Published on 2021-06-15 • Modified on 2021-06-15

In this snippet, we see a typical PHP interface. I have added comments inside about what is allowed are not. For each case, I have added the corresponding error message. This is something every PHP developer should know! The PHP version reference for this snippet is 7.4.

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<?php

declare(strict_types=1);

namespace App\Samples;

// use App\Controller\PaginationTrait; // cannot use traits inside interfaces ❌

use Symfony\Component\HttpKernel\HttpKernelInterface;

/**
 * The PHP interfaces cheatsheet: PHP 7.4.
 */
interface MyHttpKernelInterface extends HttpKernelInterface // interfaces can extend other interfaces ✅
{
    // use PaginationTrait; // cannot use traits inside interfaces ❌
    /**
     * Error= "Compile Error: Cannot use traits inside of interfaces. PaginationTrait is used in App\Samples\MyHttpKernelInterface".
     */
    public const MY_CONST = '2'; // constants are allowed ✅

    // public const MAIN_REQUEST = false; // Throws an error if the constant is alteady delcared in the parent interface:
    /**
     * Error: "Compile Error: Cannot inherit previously-inherited or override constant MAIN_REQUEST from interface Symfony\Component\HttpKernel\HttpKernelInterface".
     */

    // public static $toto = 'foo'; // public static member not allowed ❌
    /**
     * Error : Compile Error: Interfaces may not include member variables.
     */

    // protected function writePrivateProtected(string $key, string $content): int; // private / protected method declaration NOT allowed ❌
    /**
     * Error: "Compile Error: Access type for interface method App\Samples\MyHttpKernelInterface::writePrivateProtected() must be omitted".
     */

    // public function handle(); // cannot change the signature of a method of an extended interface ❌

    /**
     * Error: "Compile Error:
     *  Declaration of App\Samples\MyHttpKernelInterface::handle($message, array $context = Array, int $myIntrefaceParam = 2)
     *  must be compatible with
     *  Symfony\Component\HttpKernel\HttpKernelInterface::handle(Symfony\Component\HttpFoundation\Request $request, int $type = self::MAIN_REQUEST, bool $catch = true).
     */
    public function write(string $key, string $content): int; // public method declaration allowed ✅

    public function exists(string $key): bool; // ✅

    // Interfaces cannot have body ❌
    /*
    public function bodyNotAllowed(string $key)
    {
    }
    */
    /*
     * Error: "Compile Error: Interface function App\Samples\MyHttpKernelInterface::bodyNotAllowed() cannot contain body"
     */
}

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