It really depends on where your application stores the session. If you did not manually override the session handler in your project, it will by default use the path that is in your php.ini file. To find the php.ini file:
$ find / -name php.ini
On CentOS, it is usually located in /etc/php.ini.
php.ini ( /tmp PHP-FPM CentOS):
session.save_path = "/var/lib/php/session"
:
$ rm -Rf /var/lib/php/session/*
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