I follow the document on this page https://xdebug.org/docs/stack_trace
Under array xdebug_get_monitored_functions() section.

The code is.

<?php
/* Start the function monitor for strrev and array_push: */
xdebug_start_function_monitor( [ 'strrev', 'array_push' ] );

/* Run some code: */
echo strrev("yes!"), "\n";

echo strrev("yes!"), "\n";

var_dump(xdebug_get_monitored_functions());
xdebug_stop_function_monitor();

The result is Fatal error: Call to undefined function xdebug_start_function_monitor()