How to implement the Service Locator design pattern in PHP for centralized object retrieval?

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class ServiceLocator {
  private $services = array();

  public function __construct() {
    // Instantiate any required services
    $this->services['db'] = new Database();
    $this->services['mailer'] = new Mailer();
  }

  public function getService($serviceName) {
    if (!isset($this->services[$serviceName])) {
      throw new Exception('Service not found: ' . $serviceName);
    }
    return $this->services[$serviceName];
  }
}

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class Database {
  // ...
}

class Mailer {
  // ...
}

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$locator = new ServiceLocator();
$db = $locator->getService('db');
$mailer = $locator->getService('mailer');