Inspired by the cool REPL thing at https://github.com/bodil/hipster
Technically, this isn't even a REPL, it's just a shortcut to send some code to the PHP interpreter and get the result inside the current buffer.
Hitting <leader>p will send all text up to the current line to the
PHP interpreter. The result will be inserted in a comment beneath
the current line.
There are still a lot of things missing before this plugin can be considered usable, but it solved my current problems.