Forward environment variables to spawned processes in WP_CLI::launch() - #6166

Summary

This PR ensures that WP_CLI::launch() forwards environment variables to the spawned process so that child processes can still access configuration defined via getenv() / $_ENV (for example DB_NAME, DB_USER, and DB_PASSWORD inside wp-config.php).

Previously, WP_CLI::launch() always called Process::create( $command ) without passing an environment array. In environments where WordPress DB credentials (and other settings) are provided exclusively through environment variables (common in Docker setups), a child process spawned via WP_CLI::launch() would not see those variables, leading to database connection errors similar to issue #5362.

What this change does

• Reads the current environment from $_ENV.
• When $_ENV is not empty, calls:
  • Process::create( $command, null, $env );
    so the spawned process inherits the parent environment.
• When $_ENV is empty, falls back to the previous behaviour:
  • Process::create( $command );
    to avoid affecting setups that rely on default environment propagation.

Rationale and backward compatibility

• Fixes the scenario described in Database connection error caused by unset env vars in process #5362 where a WP_CLI::launch_self() call (and other usage of launch()) would fail with “Error establishing a database connection” because DB_* variables were not passed to child processes.
• Maintains backward compatibility by:
  • Only passing an explicit env array when $_ENV is non-empty.
  • Preserving previous behaviour for systems where $_ENV is empty.

Testing

Manual testing performed inside a Docker-based environment:

1. Installed WordPress using env-based DB configuration (DB_NAME, DB_USER, DB_PASSWORD, etc.) and wp-config.php reading them via getenv().

2. Confirmed core commands behave normally:

   export HTTP_HOST=localhost
   /wp-cli-src/bin/wp core version --allow-root
   /wp-cli-src/bin/wp plugin list --allow-root
   /wp-cli-src/bin/wp db tables --allow-root

   All commands executed successfully.

3. Verified that child processes now propagate the environment correctly, resolving the issue outlined in Database connection error caused by unset env vars in process #5362.

Related issue

• Fixes Database connection error caused by unset env vars in process #5362