Answer by Grok 3

To switch your WordPress installation from a local MySQL server to Google Cloud SQL, you need to modify the database connection settings in the wp-config.php file. This file, located in the root directory of your WordPress installation, defines how WordPress connects to its database. Below, I’ll outline the key changes required, focusing on the most straightforward and recommended approach for an external Linux server like yours.

Key Configuration Changes

The primary change involves updating the database connection parameters in wp-config.php. These are defined using PHP define() statements and include:

DB_NAME: The name of the database.
DB_USER: The username for database access.
DB_PASSWORD: The password for the database user.
DB_HOST: The host address where the database is located.

When replacing a local MySQL server (where DB_HOST is typically localhost or 127.0.0.1) with Google Cloud SQL, the DB_HOST value must be updated to point to the new database location. Additionally, if the database name, username, or password differ in your Google Cloud SQL setup, those values must be updated as well. However, if you migrate your existing database to Cloud SQL with the same name and credentials, only DB_HOST will change.

Recommended Approach: Using the Cloud SQL Proxy

For an external Linux server connecting to Google Cloud SQL, Google recommends using the Cloud SQL Proxy. The proxy simplifies the connection process and enhances security by handling authentication and encryption without requiring manual SSL certificate management. Here’s how it affects wp-config.php:

Install and Configure the Cloud SQL Proxy:
- Download and install the proxy on your Linux server.
- Authenticate it using a Google Cloud service account key with permissions to access your Cloud SQL instance.
- Run the proxy to create a local endpoint (either a TCP port or a Unix socket) that connects securely to your Cloud SQL instance.
Update DB_HOST in wp-config.php:
- Using a TCP Port: If you configure the proxy to listen on a TCP port (e.g., 3307 to avoid conflicts with a local MySQL instance), update DB_HOST to reflect this. For example:
  
  php
```
define('DB_HOST', '127.0.0.1:3307');
```
  Replace 3307 with the port number you specify when starting the proxy (e.g., ./cloud_sql_proxy -instances==tcp:3307 -credential_file=/path/to/key.json).
- Using a Unix Socket: If you configure the proxy to use a Unix socket (e.g., /cloudsql/), set DB_HOST to the socket path. For example:
  
  php
```
define('DB_HOST', '/cloudsql/<INSTANCE_CONNECTION_NAME>');
```
  Start the proxy with a command like ./cloud_sql_proxy -instances==unix:/cloudsql/ -credential_file=/path/to/key.json. Here, is the unique identifier of your Cloud SQL instance (e.g., project-id:region:instance-id).
Verify Other Settings:
- Ensure DB_NAME, DB_USER, and DB_PASSWORD match the database and user credentials you set up in Google Cloud SQL. If you exported your local MySQL database and imported it into Cloud SQL with the same name and credentials, these values can remain unchanged. Otherwise, update them accordingly:
  
  php
```
define('DB_NAME', 'your_database_name');
define('DB_USER', 'your_database_user');
define('DB_PASSWORD', 'your_database_password');
```

Alternative Approach: Direct Connection via Public IP

If you choose not to use the Cloud SQL Proxy, you can connect directly to the Cloud SQL instance using its public IP address. However, this method is less secure unless configured with SSL and requires authorizing your server’s IP address in the Cloud SQL console. In this case:

Set DB_HOST to the public IP address of your Cloud SQL instance:

php
```
define('DB_HOST', 'YOUR_CLOUD_SQL_PUBLIC_IP');
```
Optionally, configure SSL by adding certificate paths (downloaded from the Cloud SQL console) if you enable SSL enforcement:

php
```
define('DB_SSL', true);
define('MYSQL_SSL_CA', '/path/to/ca-cert.pem');
define('MYSQL_SSL_CERT', '/path/to/client-cert.pem');
define('MYSQL_SSL_KEY', '/path/to/client-key.pem');
```
Note that WordPress supports SSL connections via PHP’s MySQL extensions, but this requires additional setup and is less common than using the proxy.

What Must Be Modified?

At a minimum, you must modify DB_HOST in wp-config.php to point to your Google Cloud SQL instance, either via the proxy’s local endpoint (recommended) or the instance’s public IP (less preferred). If your database name, username, or password differ from the local MySQL setup, update those as well. Assuming you migrate the database with identical credentials, the change is limited to DB_HOST.

Final Recommendation

For security and simplicity, use the Cloud SQL Proxy with a Unix socket or a unique TCP port. For example:

With a Unix socket:

php

define('DB_HOST', '/cloudsql/<INSTANCE_CONNECTION_NAME>');

With a TCP port (e.g., 3307):

php
```
define('DB_HOST', '127.0.0.1:3307');
```

Ensure the proxy is running and properly authenticated, stop the local MySQL server if it conflicts (e.g., on port 3306), and test the connection after updating wp-config.php. This approach minimizes changes to your WordPress configuration while ensuring a secure connection to Google Cloud SQL.

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