Swap for deprecated /e argument

The `/e` modifier was deprecated in PHP55, leading to the following error being thrown in some environments:

```
Deprecated: preg_replace(): The /e modifier is deprecated, use preg_replace_callback instead in /srv/chunksite/public/forum/core/lib/ETSQLQuery.class.php on line 662
```

This fixes that, using an anonymous function rather than an evaluation of the string.