get_the_terms, whether I should use string or array as the second parameter?

Question

When I running Query Monitor on one of my page, I get a notice "Array to string conversion" for the following line:

      $terms = get_the_terms( $post->ID , array( 'product-categories') );

I then check the document for get_the_terms at https://developer.wordpress.org/reference/functions/get_the_terms/, the second parameter is a string.

However, after reading further, I find some samples in the above link and some are using string directly, like below:

$terms = get_the_terms( $post->ID , array( 'teams_positions') );

but some uses an array, like below:

$post_categories = get_the_terms( $post->ID, 'category' );

So I am totally confused.

Answer 1

Going by the documentation it should only be string.

Notice this line $terms = get_object_term_cache( $post->ID, $taxonomy );

The get_object_term_cache( int $id, string $taxonomy ) always expects the second (required) argument to be a string, so if you pass an array as the second argument to get_the_terms you will get the Array to string conversion notice.

Second argument should always be a string, those example could be old and the function was probably updated in newer versions.

For further information I would check wordpress change logs in the versions, maybe there you will find this function with a second parameter that used to accept array as well as string.