Extended classes cannot be located by Xdebug in PhpStorm.

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There could be several reasons why extended classes are not being located by Xdebug in PhpStorm:

1. Make sure that the class files are included in the project. PHPStorm might not be able to locate the files if they are not present in the project.

2. Check the configuration of Xdebug in PhpStorm. Ensure that the remote debugging is enabled, and the port number is correct.

3. Check the PHP settings: Xdebug might not be enabled or installed as a PHP extension. To make sure, run the phpinfo() function in a script, and check if Xdebug is listed.

4. The issue might be related to the type of error being reported. For instance, if a fatal error occurs in the parent class, the debugger may not be able to continue execution and locate the child class.

5. Ensure that code is correctly versioned, updated, and matches the local environment. Even if the code is identical to the remote code, debugging can fail if the version of PHP or Xdebug differs.

6. Check if there are any breakpoints set in the parent class. If so, disable or remove them, and try again.

7. Try to reproduce the issue with a simple, minimal example to isolate the issue.

Overall, make sure that all configuration settings are correct, and that the source code is correct, to ensure that Xdebug is correctly locating the extended classes in PhpStorm.