What would be really great to have is something similar to classic pcntl_clone() linux kernel function on the PHP level, accepting a function to be executed as a parameter and sharing all the variables/state with the primary and other created threads.

Locking could be done with a simple and optional use of mutexes on the application level, if one needs to view fully consistent state of a certain variable / array. (ie. otherwise, foreach() loop while writing to an array in the other thread might not see some recently added entries)

As an idea, on the internal implementation side, copy-on-write semantics could be used with read-only operations seeing the previous version while the write/modify operation is executed on a copy.

Even if unstable at first (marked as experimental or some such), it would be a good starting point for making PHP thread-safe, re-entrant, and multi-core compatible.

How many year the PHP community will wait for <?PHP new Thread?>

I believe, that this would be a excellant addition to PHP as I've run into many situation where multi-threading would have made it much more performant than simply running everything in one thread where a single function taking it's time or other lengthy tasks can have a significant impact on the time it takes the script to run.

This is not going to happen in PHP core, but it can be done through an extension. 
One example is here: http://pecl.php.net/package/pthreads