LSP example in PHP

Liskov breach.php

LSP: "Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program."

<?php

namespace App\User;

// imagine the following interface
interface Repository
{
   public function getById($id);
}

// without parameter type-hints, return types, or even a doc-block, it is impossible for client code to be able 
// to rely on the correctness of the implementation
// a programmer trying to implement this interface will be having to guess at what to return, what to accept, etc.
// it would be very easy to inadvertently create an implementation which breaches LSP and causes problems for client code

// We can address this by adding parameter type-hints and a return type
interface Repository
{
   /**
    * @throws \App\User\Error\NotFound
    */
   public function getById(int $id): User;
}

// Note that we have also added a @throws annotation.
// According to the definition of the interface, client code should be able to reliably work as follows:

try {
  $user = $repository->getById($id);
} catch (NotFound $e) {
  // handle exception
}


// It is still possible to breach LSP even with parameter type-hints and return types in place
// The following implementation would be a breach of LSP 
// Even though it meets the interface implementation according to PHP's language-level requirements,
// it throws something which is not defined by the behaviour of the `Repository` type.
// The above client code would break because it is not expecting to need to catch an exception of type \Exception

class Database implements Repository
{
  public function getById(int $id): User
  {
    $row = $this->db->query("SELECT * FROM users WHERE id = ?", [$id]);
    
    if ($row) {
      return User::fromRow($row);
    }
    
    throw new \Exception("User not found!");
  }
}