When using PHP and Apache you would have three main error log files. The Apache (webserver) error log can be found within:
/var/log/apache2/error_logThe site/application error log can be found within:
/home/username/logs/$domain_tld.php.error.logThe above is for errors that are coming from the PHP application itself, for example, issues with the code or syntax.
The final error log is for PHP-FPM in general, in here you can find issues with max_children or errors with the service itself:
/opt/cpanel/ea-php$$/root/usr/var/log/php-fpm/error.logMake sure to replace ea-php$$ with the actual PHP version you are running, for example, ea-php73 for PHP 7.3 version.
For more information on log files and PHP-FPM, please visit the following links:
The cPanel Log Files - cPanel Knowledge Base - cPanel DocumentationPHP-FPM - Version 80 Documentation - cPanel Documentation