WordPress built-in method to extract array of term IDs?

I know you’ve long since solved this, but wanted to offer another solution. This question popped up as “related” when I was answering another one.

You can use the WordPress function wp_list_pluck to return an array with values as one of the fields of the array or objects sent to the function. In other words, send the function the objects and specify the field you want back and you’ll get an array with only that field.

For instance, you can do something like:

$ids = wp_list_pluck(get_terms('category', 'hide_empty=0'), 'term_id'));

$ids will be an array of the terms ids that you wanted to capture.