Xdebug breakpoints not hit: PhpStorm detaches with "Cannot bind file to the web server project"

Problem

Xdebug connects to PhpStorm on every request; however, the session ends before any breakpoint is sent to Xdebug.

Cause

PhpStorm connects to the web server, but cannot map the PhpStorm project listening to this debugging connection to a project on the web server.

The following lines appear in the xdebug.log (decoded from Base64):

[6260] [Step Debug] WARN: Debug client detached: Cannot bind file C:\inetpub\...\testfile.php to the web server project. For more information, refer to https://www.jetbrains.com/help/phpstorm/troubleshooting-php-debugging.html#detach-from-debug-session-reasons.

Resolution

1. Open Settings | PHP | Servers.
2. Remove all the entries from this menu.
3. Start the debugging session anew.
4. Verify the values appearing in the "Incoming connection" dialog and press OK.