about php4 compiler

In Zend.COM say that " The PHP 4.0 scripting engine - Zend Engine - uses a much more efficient "compile-then-execute" paradigm, instead of the "execute-while parsing" model employed in PHP 3.0. " Since "compile-then-execute" .. , php4 must have something like "java bytecode" in the memory .. Why not just write those "bytecode" into disk.. Next time when php execute script, only need to check the modify time of source script..and execute .... --- hjf