A PHP based API encapsulation boilerplate; Has the basic setup for a class for the API functions, the HTTP request, and the API key.

APIManager.class.php

<?php

/**
 * This is a PHP demo/boilerplate for a basic API which relies on a API key.
**/

namespace amattu\api;

/**
 * The main API request manager class
 *
 * NOTE:
 *   (1) This is where the main API functions go
 *   (2) If no errors are thrown during construction
 *       it's safe to assume the token is valid
 */
class APIManager {
  /**
   * Instance of HttpRequest
   *
   * @var HttpRequest
   */
  protected $HttpRequest;

  /**
   * Class Constructor
   *
   * @param array $request_header fields
   * @throws TypeError
   * @throws InvalidAPITokenException
   * @author Alec M. <https://amattu.com>
   * @date 2021-10-24T12:00:27-040
   */
  public function __construct(array $request_header)
  {
    try {
      $this->HttpRequest = new HttpRequest($request_header);
    } catch (InvalidAPITokenException $e) {
      // Handle invalid API token error
    } catch (InvalidRequestException $e) {
      // Handle invalid HTTP request error
    } catch (TypeError $e) {
      // Invalid type passed to a constructor
    }
  }

  /** Add more API functionality here; i.e. Fetch users, CRUD, etc **/
  /** Access token info with $this->HttpRequest->getToken() **/
}

APIToken.class.php

<?php

/**
 * This is a PHP demo/boilerplate for a basic API which relies on a API key.
**/

namespace amattu\api\token;

class InvalidAPITokenException extends \Exception {};

/**
 * API Token Encapsulation Class
 */
class APIToken {
  /**
   * A reference to the API token
   *
   * @var string
   */
  private $APIToken;

  /**
   * Class Constructor
   *
   * @param string $token
   * @throws TypeError
   * @throws InvalidAPITokenException
   * @author Alec M. <https://amattu.com>
   * @date 2021-10-24T11:55:44-040
   */
  public function __construct(string $token)
  {
    $this->APIToken = $token;

    if (!$this->isValid())
      throw new InvalidAPITokenException("Invalid API token provided");
  }

  /**
   * Determine if the API token is valid
   *
   * @return bool valid token
   * @throws None
   * @author Alec M. <https://amattu.com>
   * @date 2021-10-24T11:56:08-040
   */
  public function isValid() : bool
  {
    return true; // Add validation logic here
  }

  /** Add more functions here; i.e. Permission level, expiry, user info, etc **/
}

HttpRequest.class.php

<?php

/**
 * This is a PHP demo/boilerplate for a basic API which relies on a API key.
**/

namespace amattu\api;

class InvalidRequestException extends \Exception {};

/**
 * API HTTP Request Representation Class
 */
class HttpRequest {
  /**
   * A class instance of APIToken
   *
   * @var APIToken
   */
  private $APIToken = null;

  /**
   * A array of HTTP header fields
   *
   * @var array
   */
  private $fields = [];

  /**
   * Class Constructor
   *
   * @param array HTTP $request header fields
   * @throws TypeError
   * @throws InvalidAPITokenException
   * @throws InvalidRequestException
   * @author Alec M. <https://amattu.com>
   * @date 2021-10-24T11:56:37-040
   */
  public function __construct(array $headers)
  {
    if (!$request["bearer"])
      throw new InvalidRequestException("No bearer token found in request");

    $this->APIToken = new APIToken($request["bearer"]);
    $this->fields = $request;
  }

  /**
   * Return instance of APIToken
   *
   * @return APIToken instance of API token
   * @throws TypeError
   * @author Alec M. <https://amattu.com>
   * @date 2021-10-24T12:11:57-040
   */
  public function getToken() : APIToken
  {
    return $this->APIToken;
  }

  /** Do anything else with the request here; i.e. access more fields, additional validation, etc **/
}