I get this error when using Xdebug version 3 with PhpStorm version 2020.2.4 :
Connection was not established.
Cannot start debugger session with 'Xdebug 3.0.0' Validate installation
php.ini Config:
xdebug.mode=develop
xdebug.start_with_request=yes
xdebug.client_port=9003
Xdebug 3 is fully supported from 2020.3 version only.