conditional_admin_notice.php

<?php

/**
 * This is a snippet if your WordPress plugin uses composer autoloading.
 *
 * Instead of just dying, it will fail gracefully and add an admin notice to the backend,
 * informing the dev to run the autoloader. 
 *
 * Could also be adjusted to check for other things like a /build/ folder or similar things.
 *
 * Remember to adjust `textdomain` and the `plugin_name` to your needs.
 */

if ( ! file_exists( __DIR__ . '/vendor/autoload.php' ) ) {
	add_action(
		'admin_notices',
		static function (): void {
			wp_admin_notice(
				sprintf(
				/* translators: %1$s: composer command, %2$s: the plugin path */
					esc_html__( 'Please run %1$s in %2$s to install the required dependencies.', 'textdomain' ),
					'<code>composer install</code>',
					'<code>plugin_name</code>'
				),
				[
					'type' => 'error',
				]
			);
		}
	);

	return;
}
require_once __DIR__ . '/vendor/autoload.php';