the main reason being - php has strange scoping semantics even in the inetstines. otherwise it would've been trivial - it has a kind of garbage collection, tail calls possibly can be implemented with some arms twisting, new syntax can be added via extension modules. So R5RS is nearly doable.

But writing it in php only - somobody did a php-scheme interpreter - it is soooo slow.

Anyway, sorry for replying late. I was offline for a long time.