Custom WP-CLI command and Unit testing with PHPUnit

<?php

class CLI_Monthly_Reports extends WP_CLI_Command {

	/**
	 * Year number.
	 *
	 * @var string $year
	 */
	private $year = '';

	/**
	 * Month number.
	 *
	 * @var string
	 */
	private $month = '';

	/**
	 * Columns.
	 *
	 * @var array
	 */
	private $columns = [
		'Category',
		'Count',
	];

	/**
	 * Generate monthly article reports.
	 *
	 * [--month]
	 * : for specific month
	 *
	 * @subcommand monthly-reports
	 *
	 * @param array $args       PositionaL Arguments.
	 * @param array $args_assoc Associative Arguments.
	 */
	public function generate_reports( array $args, array $args_assoc = [] ): void {
		$this->year  = gmdate( 'Y' );
		$this->month = gmdate( 'm' );

		if ( ! empty( $args_assoc['month'] ) ) {
			$this->month = $args_assoc['month'];
		}

		$categories      = [];
		$categories = get_categories( [ 'hide_empty' => false ] );
		if ( empty( $categories ) ) {
			WP_CLI::Error( 'No categories found.' );
		}

		\WP_CLI\Utils\format_items(
			'table',
			$this->format_data( $categories ),
			$this->columns
		);
	}

	/**
	 * Format categories.
	 *
	 * @param array $categories
	 *
	 * @return array
	 */
	private function format_data( array $categories ): array {
		$format = [];

		foreach ( $categories as $category ) {
			array_push(
				$format,
				[
					$this->columns[0] => $category->name,
					$this->columns[1] => $this->get_post_cout_by_category_id( $category->term_id ),
				]
			);
		}

		return $format;
	}

	/**
	 * Get Post count by category.
	 *
	 * @param int $id Category Id.
	 *
	 * @return int
	 */
	private function get_post_cout_by_category_id( int $id ): int {
		$args      = [
			'post_type'      => 'post',
			'posts_per_page' => 1,
			'year'           => esc_attr( $this->year ),
			'monthnum'       => esc_attr( $this->month ),
			'cat'            => esc_attr( $id ),
		];
		$the_query = new WP_Query( $args );

		return $the_query->found_posts;
	}
}
WP_CLI::add_command( 'd9_cli', 'CLI_Monthly_Reports' );

/**
* Bootstrap the CLI dependencies
*
* This is important to test the CLI classes.
*/
if ( ! defined( 'WP_CLI_ROOT' ) ) {
	define( 'WP_CLI_ROOT', '/path/to/vendor/wp-cli/wp-cli' );
}

include WP_CLI_ROOT . '/php/utils.php';
include WP_CLI_ROOT . '/php/dispatcher.php';
include WP_CLI_ROOT . '/php/class-wp-cli.php';
include WP_CLI_ROOT . '/php/class-wp-cli-command.php';

\WP_CLI\Utils\load_dependencies();

<?php
/**
 * Class Test_CLI_Monthly_Reports
 */

/**
 * Tests for CLI_Monthly_Reports class
 */
class Test_CLI_Monthly_Reports extends WP_UnitTestCase {

	/**
	* Test for generate reports on current month.
	*/
	public function test_generate_report_for_current_month() {
		$monthly_report = new CLI_Monthly_Reports();

		$output = '+---------------+-------+
| Category      | Count |
+---------------+-------+
| Uncategorized | 1     |
+---------------+-------+
';
		$monthly_report->generate_reports( [] );
		$this->expectOutputString( $output );
	}

	/**
	* Test for generate reports on previous month.
	*/
	public function test_generate_report_for_previous_month() {
		$monthly_report = new CLI_Monthly_Reports();

		$output = '+---------------+-------+
| Category      | Count |
+---------------+-------+
| Uncategorized | 0     |
+---------------+-------+
';
		$monthly_report->generate_reports( [], [ 'month' => gmdate( 'm', strtotime( 'previous month' ) ) ] );
		$this->expectOutputString( $output );
	}
}