How It works:

• Prefix all data sent to EvalWorker with a 1 byte command identifier can be either CMD_EVAL ("\0") or CMD_COMPLETE ("\1").
• completiion function sends CMD_COMPLETE . substr($info['line_buffer'], 0, $info['end']) to eval worker
• eval worker sends back completions as a JSON blob.

Completion function inspired from https://github.com/erikdubbelboer/php-repl/blob/master/repl.php

Remove some dead code, keep eval worker alive if evaluation fails.