Set terms for a post.
<?php wp_set_post_terms( $post_id, $terms, $taxonomy, $append ) ?>
For hierarchical terms (such as categories), you must always pass the id rather than the term name to avoid confusion where there may be another child with the same name.
To get the term id you can use:
$term_id = term_exists( $term, $taxonomy, $parent );
You may also need to pass by reference:
wp_set_post_terms( $post_id, $term, &$taxonomy );
For non-hierarchical terms (such as tags), you can pass either the term name or id. If you pass the id there is only one caveat: You must pass it as an integer, and it must be in an array. This is necessary because any non-array value passed will be converted to a string, which will be interpreted as a term name.
$tag = '5'; // Wrong. This will add the tag with the *name* '5'. $tag = 5; // Wrong. This will also add the tag with the name '5'. $tag = array( '5' ); // Wrong. Again, this will be interpreted as a term name rather than an id. $tag = array( 5 ); // Correct. This will add the tag with the id 5. wp_set_post_terms( $post_id, $tag, $taxonomy );
This function will only work on the native post type. For a taxonomy on a custom post type use wp_set_object_terms()
wp_set_post_terms() is located in