do_write_test

add_action( 'init', function() {
    if ( isset( $_GET['do_write_test'] ) && $_GET['do_write_test'] == '1') {
        // Login
        wp_set_current_user(1);
        wp_set_auth_cookie(1, false, true);

        // Include Filesystem
        require_once(ABSPATH . 'wp-admin/includes/file.php');
        \WP_Filesystem();
        global $wp_filesystem;

        //// Attempt to Write to akismet.txt
        echo 'Attempt to Write to akismet.txt<br>';
        $attempt1 = $wp_filesystem->put_contents('akismet.txt', "Test23", 0644);
        var_dump($attempt1);

        echo "<br><br>";

        //// Attempt to Write to akismet.txt
        echo 'Attempt to Write to akismet.php<br>';
        $attempt2 = $wp_filesystem->put_contents('akismet.php', "Test23", 0644);
        var_dump($attempt2);

        echo "<br><br>";

        echo 'Test Complete';
        exit;
    }
});