PHP 7, Symfony 3: Fatal error 1 abstract method and must therefore be declared abstract or implement the remaining methods

Question

After a php update from 5.6 to 7, Symfony 3 raise this exception:

Fatal error: Class Symfony\\Component\\HttpFoundation\\Session\\Storage\\Proxy\\Se

Answer 1

Restarting the httpd services can clear this error.

Have not been able to reproduce the error after experiencing it but it could be a PHP crash that in turn causes an incomplete state that makes the error message get stuck in the buffer response of the httpd service, it is then served for every single request to any vhost being served on that service from then on until the service is killed.

Credit Scott Flacks comment in the other answer.