wp_set_post_categories() WP 1.0

Set categories for a post.

If the post categories parameter is not set, then the default category is going used.

No Hooks.

Return

Array/false/WP_Error. Array of term taxonomy IDs of affected categories. WP_Error or false on failure.

Usage

wp_set_post_categories( $post_ID, $post_categories, $append );

$post_ID(int)

The Post ID. Does not default to the ID of the global $post.
Default: 0

$post_categories(array/int)

List of category IDs, or the ID of a single category.
Default: empty array

$append(true/false)

If true, don't delete existing categories, just add on. If false, replace the categories with the new categories.
Default: false

Code of wp set post categories: wp-includes/post.php ^{WP 5.2.1}

<?php
function wp_set_post_categories( $post_ID = 0, $post_categories = array(), $append = false ) {
	$post_ID     = (int) $post_ID;
	$post_type   = get_post_type( $post_ID );
	$post_status = get_post_status( $post_ID );
	// If $post_categories isn't already an array, make it one:
	$post_categories = (array) $post_categories;
	if ( empty( $post_categories ) ) {
		if ( 'post' == $post_type && 'auto-draft' != $post_status ) {
			$post_categories = array( get_option( 'default_category' ) );
			$append          = false;
		} else {
			$post_categories = array();
		}
	} elseif ( 1 == count( $post_categories ) && '' == reset( $post_categories ) ) {
		return true;
	}

	return wp_set_post_terms( $post_ID, $post_categories, 'category', $append );
}

Related Functions

From category: For posts