PHP does not inherently "belong" to a specific Linux user group in the same way a user account belongs to a group. Instead, the user and group under which PHP processes run are determined by the configuration of the web server or PHP-FPM (FastCGI Process Manager) that executes PHP scripts.
Commonly, PHP processes run under the user and group associated with the web server:

   Apache:
       On Debian/Ubuntu, the user and group are typically www-data:www-data.
       On CentOS/RedHat, the user and group are typically apache:apache.
   Nginx with PHP-FPM:
   The user and group for PHP-FPM pools are often configured to be www-data:www-data or a dedicated user and group for each website (e.g., user1:user1).

The specific user and group can be found by examining the web server's or PHP-FPM's configuration files (e.g., Apache's envvars file, PHP-FPM's pool configuration files like /etc/php-fpm.d/www.conf). You can also determine the user by running a PHP script containing <?php echo shell_exec('whoami'); ?>.