parallel requires a build of PHP with ZTS (Zend Thread Safety) enabled (--enable-zts, or on non-Windows systems prior to PHP 8.0.0, --enable-maintainer-zts)
Zend Thread Safety cannot be enabled post build; it is a build time configuration option.
parallel should build anywhere there is a working Posix Threads header (pthread.h) and ZTS build of PHP, including Windows (using the pthread-w32 project from redhat).
parallel releases are hosted by PECL and the source code by » github, the easiest route to installation is the normal PECL route: » https://pecl.php.net/package/parallel.
Windows users can download prebuilt release binaries from the » PECL website.
Windows users need to take the additional step of adding pthreadVC2.dll (distributed with Windows releases) to their PATH.