I have set up:

• an auto_prepend_file, in eg. /etc/php/8.1/fpm/php.ini
• and error_prepend_string, in that prepend file: ini_set('error_prepend_string', '<!-- TEST_ERROR_STRING -->');

When xdebug is off, and I run the code $x = new NoTAClass();, I get the expected error prepend string.

When xdebug is on, and I run the code $x = new NoTAClass();, I do not get the expected error prepend string any more.

Note that, with xdebug on, the error prepend string is shown as expected for non-fatal errors. such as for $x = $undefinedVar;