It began with a simple bit of laziness: I wanted a preprocessor so that I could write as if PHP had multiple return values. I'd write "return $x, $y;" in the callee, and "$a, $b = fn();" in the caller, and the preprocessor would rewrite it to valid PHP (throwing array() and list() around the appropriate expressions).
But I'm even too lazy for that. To do this right, I'd need to fully parse the PHP, so I could understand more complicated return expressions like method calls. So instead of that, I slapped together a lexer for s-expressions. They're a lot less hairy, and this is just some twisted experiment.
I was halfway through putting together a parser this evening for the lexer output, when I realized: a few years ago, I ported the metacircular evaluator from the SICP lectures into Ruby... then discovered I would need to write an s-expression parser, which you get for free with Lisp. (That project then died.) But if I finish an s-expression parser... I can port the metacircular evaluator to it and have the world's stupidest Lisp-1 implementation, i.e. it'll be done in PHP.*
Alternatively, I can define a package in SBCL that emits PHP, and have the reader and macros for free. Then my head exploded.
* Because this tool was intended for PHP shops, the compiler would have to be written in and emit PHP so there's no Scary Foreign Language involved, other than the compiler's input. And originally, that input language was going to be almost PHP.