This » PECL extension is not bundled with PHP.
Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: » https://pecl.php.net/package/luasandbox.
If the operating system is Debian 9 or later, or Ubuntu 18.04 or later, then LuaSandbox should typically be installed from the package php-luasandbox:
sudo apt-get install php-luasandbox
Debian 9 users may need to add
deb http://ftp.debian.org/debian stretch-backports main