Custom language injection in markdown

Hi !

I'm trying to configure custom language injections for markdown, such as this syntax:

```php
echo "This is PHP code";
```

The markdown extension added the language to the list for me, so that is fine. But I have failed a bit at finding suitable examples for such a mapping... I have the regexp to match the block (```php\n((.*\n)+?)```), but i'm not even sure where it should go. Whatever I've tried, the checkbox for my injection remains uncheckable.