Dependency Injection.

DatabaseConfiguration.php

<?php

namespace DesignPatterns\Structural\DependencyInjection;

/**
 * Class DatabaseConfiguration
 *
 * @package DesignPatterns\Structural\DependencyInjection
 */
class DatabaseConfiguration {

  /**
   * @var string
   */
  private $host;

  /**
   * @var int
   */
  private $port;

  /**
   * @var string
   */
  private $username;

  /**
   * @var string
   */
  private $password;

  /**
   * DatabaseConfiguration constructor.
   *
   * @param string $host
   * @param int $port
   * @param string $username
   * @param string $password
   */
  public function __construct(string $host, int $port, string $username, string $password) {
    $this->host = $host;
    $this->port = $port;
    $this->username = $username;
    $this->password = $password;
  }

  /**
   * @return string
   */
  public function getHost(): string  {
    return $this->host;
  }

  /**
   * @return int
   */
  public function getPort(): int {
    return $this->port;
  }

  /**
   * @return string
   */
  public function getUsername(): string {
    return $this->username;
  }

  /**
   * @return string
   */
  public function getPassword(): string {
    return $this->password;
  }

}

DatabaseConnection.php

<?php

namespace DesignPatterns\Structural\DependencyInjection;

/**
 * Class DatabaseConnection
 *
 * @package DesignPatterns\Structural\DependencyInjection
 */
class DatabaseConnection {

  /**
   * @var DatabaseConfiguration
   */
  private $configuration;

  /**
   * DatabaseConnection constructor.
   *
   * @param \DesignPatterns\Structural\DependencyInjection\DatabaseConfiguration $config
   */
  public function __construct(DatabaseConfiguration $config) {
    $this->configuration = $config;
  }

  /**
   * @return string
   */
  public function getDsn(): string {
    return sprintf(
      '%s:%s@%s:%d',
      $this->configuration->getUsername(),
      $this->configuration->getPassword(),
      $this->configuration->getHost(),
      $this->configuration->getPort()
    );
  }

}

index.php

<?php

namespace DesignPatterns\Structural\DependencyInjection;

require_once 'DatabaseConfiguration.php';
require_once 'DatabaseConnection.php';

$dbconfig = new DatabaseConfiguration('localhost', '3306', 'root', 'root');
$dbconnection = new DatabaseConnection($dbconfig);
$test = $dbconnection->getDsn();
var_dump($test);