c. Making a Request:
Headers:

Add the Authorization header with the Bearer token.

makefile
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Authorization: Bearer your_token_here
POST Parameters:

username: The user's username.
password: The user's password.
Example using curl:

bash
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curl -X POST \
  -H "Authorization: Bearer your_token_here" \
  -d "username=johndoe&password=password123" \
  https://yourdomain.com/api_authenticate.php
d. Handling Responses:
Success (HTTP 200):

Receives a JSON object with user data, accounts, and addresses.
Failure:

Receives a JSON object with an error_code and message.
6. Enumerated Error Codes:
Error Code 501:

Condition: Authorization header missing.
HTTP Status Code: 401 (Unauthorized).
Message: "Authorization header missing."
Error Code 502:

Condition: Invalid Authorization header format.
HTTP Status Code: 401 (Unauthorized).
Message: "Invalid Authorization header format."
Error Code 503:

Condition: Invalid or inactive token.
HTTP Status Code: 401 (Unauthorized).
Message: "Invalid or inactive token."
Error Code 504:

Condition: Missing username or password.
HTTP Status Code: 400 (Bad Request).
Message: "Missing username or password."
Error Code 505:

Condition: Invalid username or password.
HTTP Status Code: 401 (Unauthorized).
Message: "Invalid username or password."
Error Code 506:

Condition: Database error occurred.
HTTP Status Code: 500 (Internal Server Error).
Message: "Database error occurred."
Error Code 507:

Condition: An unexpected error occurred.
HTTP Status Code: 500 (Internal Server Error).
Message: "An unexpected error occurred."
7. Additional Recommendations:
a. Security Enhancements:
HTTPS:

Ensure the API is accessed over HTTPS to secure data in transit.
Token Revocation:

Implement a mechanism to revoke tokens when necessary.
Rate Limiting:

Protect your API from abuse by limiting the number of requests.
b. API Design Improvements:
RESTful Principles:

Consider designing the API following RESTful principles for better scalability and maintainability.
Versioning:

Implement API versioning to manage updates without breaking existing integrations.
c. Error Handling:
Consistent Error Structure:

Ensure all error responses follow a consistent format.
Localization:

If needed, provide error messages in multiple languages.
d. Logging and Monitoring:
Detailed Logging:

Implement detailed logging for monitoring and debugging purposes.
Monitoring Tools:

Use monitoring tools to keep track of API performance and errors.
8. Conclusion:
The provided PHP script meets your requirements by:

Utilizing your existing database connection include file.
Implementing Bearer token authentication via the Authorization header.
Providing detailed error codes for different failure scenarios.
Next Steps:

Set up the necessary database tables and ensure your data is in place.
Test the API thoroughly to ensure it behaves as expected.
Implement additional security measures as recommended.
If you have any questions or need further assistance with implementing or customizing this script, please feel free to ask!