Why PHP has separate version of ts(thread safe)/nts(non thread safe) while it doesn't support multi-threading?
While you can't spawn threads from PHP code you can use PHP with a multi-threaded web server that handles concurrent requests on different threads. In this case the TS (thread-safe) version of PHP should be used.
The TS version of PHP keeps the state of each request in its own memory location. This is necessary because all requests in a multi-threaded server share the same address space.
The alternative is to use a multi-process (usually prefork) server. With such a server some state can be kept in global variables without affecting concurrent requests. That's how the NTS (non thread-safe) version of PHP is implemented.