If you use any control panel such as cPanel, Plesk or DirectAdmin , there is no extra configuration required since LSWS installation script will configure everything for you and you should be able to run PHP out of the box after installation. LSWS will use the same PHP version as Apache set on your control panel automatically.
You can refer to the corresponding section for your environment to understand how it works for
If you are using CloudLinux, PHP selector is the recommended way to set up PHP.
Unlike apache or nginx using php-fpm or fastcgi, LSWS uses LSPHP, which is faster than php-fpm or fastcgi. On LSWS (native), you can install LSPHP from LiteSpeed CentOS, Ubuntu/Debian repositories, then create/configure external app and script handler to point LSWS to use the newly installed PHP binaries.
Please see Configuring LiteSpeed Web Server for PHP.
Please see suEXEC or non-suEXEC, a guide to figure out which PHP execution method you are using.
As of LiteSpeed Web Server version 5.3, LSPHP can be run independent of the web server in Detached mode. This allows the LSPHP process, and it's opcode cache, to survive through server restarts.
Check the availability of LSPHP PECL extention or build by yourself? See How to install pecl extention for LSPHP.