WordPress: Add Another Category To Posts

add-another-category-to-posts.php

<?php

/**
 * Takes posts that are in category A and also adds them to category B
 */

// Uncomment this to proceed
exit( "You need to edit the file and enter the hostname and category IDs.\n" );

// What blog? Asking for both for safety reasons.
$target_blog_id = 0;
$_SERVER['HTTP_HOST'] = 'site.com'; // Mapped domain

// What's the category ID to pull posts from?
$source_category = 123;

// What's the category ID to add?
$add_category = 456;

# Okay, stop editing! :)

function free_up_memory() {
	global $wpdb, $wp_object_cache;
	$wpdb->queries = array(); // or define( 'WP_IMPORTING', true );
	if ( ! is_object( $wp_object_cache ) )
		return;
	$wp_object_cache->group_ops = array();
	$wp_object_cache->stats = array();
	$wp_object_cache->memcache_debug = array();
	$wp_object_cache->cache = array();
	$wp_object_cache->__remoteset(); // important
}

define( 'WP_IMPORTING', true );
define( 'SAVEQUERIES', false );
set_time_limit( 0 );
ini_set( 'memory_limit', '768M' );

require_once( dirname( __FILE__ ) . '/../../wp-load.php' );

switch_to_blog( $target_blog_id );

// Safety check
$blog_url = 'http://' . $_SERVER['HTTP_HOST'];
if ( $blog_url != home_url() )
	exit( "Blog ID {$target_blog_id} is " . home_url() . ", not {$blog_url}. Please check the blog ID and the HTTP_HOST.\n" );

// Build query args
$posts_at_a_time = 50;
$args = array(
	'category__in' => array( $source_category ),

	'posts_per_page' => $posts_at_a_time,
	'offset' => 0,

	// Prevent new posts from affecting the order
	'orderby' => 'ID',
	'order' => 'ASC',

	// Speed this up
	'no_found_rows' => true,
	'update_post_meta_cache' => false,
	'update_post_term_cache' => false,
);

// Get the first set of posts
$query = new WP_Query( $args );

$found_posts = 0;

// Start looping through chunks of posts
while ( $query->have_posts() ) {

	foreach ( $query->posts as $post ) {
		echo '.';
		$found_posts++;

		wp_set_object_terms( $post->ID, array( $add_category ), 'category', true );
	}

	free_up_memory();

	// Get more posts to process
	$args['offset'] = $args['offset'] + $posts_at_a_time;
	$query = new WP_Query( $args );
}

echo "\nAll done! Updated {$found_posts} posts.\n";