It is possible de modify certain PHP parameters using directives inside an ".htaccess" file specific to your sites. To affect the whole site, lines of code can be added to a file that is named ".htaccess" (without the quotes) at the root of your website.
Therefore, if you need to change a specific parameter, such as max_input_vars, it is possible to add the following lines:
# PHP 8.1 <IfModule mod_php.c> php_value max_input_vars 10000 </IfModule>
# PHP 7.3 <IfModule mod_php7.c> php_value max_input_vars 10000 </IfModule>
To verify that the changes have taken effect, it is possible to verify on a page that outputs the PHP configuration information. You can, for example, create a page called "test.php" which contains the following lines:
<?php phpinfo(); ?>
Certain content management systems provide such pages to ease that task. Check the documentation of your CMS to find out if such a page is available.