I don't understand what "multiple cores instead of multiple threads" means,anyways: PHP's purpose is to handle web requests in a shared nothing architecture. By that architecture it can be easily be scaled linearly vertically (over multiple machines) as well as horizontally (bigger machines= easily and thus use all available CPU power.

Threading inside individual scripts in this context brings more complications and less benefit. PHP offers asynchronous operations and we will increase that, where it makes sense. To provide a good user experience the suggested approach is to offload CPU-intensive tasks to background workers using gearman or rabbitmq or such.

For experimental use of threads from within a script one can look into http://pecl.php.net/pthreads The core distribution doesn't provide this on purpose.