With the above set up, I add the following to the top of the class that holds the function I want to trace:

ini_set('xdebug.output_dir', 'C:/dev-server/www/opg_code/data/misc');
ini_set('xdebug.trace_output_name', 'covercraft.postback');
ini_set('xdebug.collect_assignments', 1);
ini_set('xdebug.collect_return', 1);
ini_set('xdebug.var_display_max_children', 32);
ini_set('xdebug.var_display_max_data', 1024);
ini_set('xdebug.var_display_max_depth', 6);

I put xdebug_start_trace() at the top pf the function I want to trace and xdebug_stop_trace() before the return statement.

I had the following at the end of my php.ini:

zend_extension = xdebug
xdebug.mode = trace
xdebug.use_compression = false
xdebug.log = C:\dev-server\logs\xdebug.log

Ran the app, no trace file was generated. xdebug log said:

[22876] [Config] INFO: Trigger value for 'XDEBUG_TRIGGER' not found, falling back to 'XDEBUG_TRACE'
[22876] [Config] INFO: Trigger value for 'XDEBUG_TRACE' not found, so not activating

Added 'XDEBUG_TRACE=1' to query string, ran the app again, and no trace file was generated

Added 'xdebug.start_with_request = yes' to php.ini, ran the app again, no trace file.

Added 'xdebug.output_dir = C:\dev-server\www\opg_code\data\misc' to php.ini, ran app again, now get trace file of the app loading*, but still no trace file for the function I want to trace.

(*the app was built in Zend Framework 2, and has been updated over the last few years to keep up with changes to PHP)