If you have installed Phiki inside of a Statamic application, then you can use the CommonMark extension to highlight code blocks in your Markdown content. You can enable the extension by registering it with Statamic’s Markdown class inside the boot method of a service provider. 
use Statamic\Facades\Markdown;
use Phiki\Adapters\CommonMark\PhikiExtension;
use Phiki\Theme\Theme;
use Phiki\Phiki;

class AppServiceProvider extends ServiceProvider
{
    public function boot(): void
    {
        Markdown::addExtension(fn () => new PhikiExtension(Theme::GithubLight, resolve(Phiki::class)));
    }
}
For more information on using the PhikiExtension, see the CommonMark documentation. If you wish to customize Phiki further, you can also refer to the Laravel documentation since Statamic is built on top of Laravel.