Bulk move and delete Categories in WordPress

category_mover.php

<?php
if ( ! defined( 'PHP_SAPI' ) || PHP_SAPI !== 'cli' ) {
	exit( 'You shall not pass' );
//	echo '<pre>';
//	set_time_limit( 0 );
}

$dir = $argv[1] ?? __DIR__;

// Load wp-config to make sure we can use WordPress.
if ( ! is_readable( $dir . '/wp-config.php' ) ) {
	exit( 'Could not find wp-config, please add the correct path to the wp-config.php to the cli command' );
}

require_once $dir . '/wp-config.php';

if ( ! defined( 'ABSPATH' ) ) {
	exit( 'You shall not pass' );
}

if ( ob_get_level() ) {
	echo ob_get_flush(); // phpcs:ignore HM.Security.EscapeOutput.OutputNotEscaped
}

$mapping = [
  // Dublicate => correct category
  1 => 2,
  3 => 4,
];


foreach ( $mapping as $duplicate_category => $correct_category ) {
	echo 'Updating ' . $duplicate_category . PHP_EOL;
	$args = [
		'numberposts' => -1,
		'category' => $duplicate_category,
	];

	$posts = get_posts( $args );

	echo 'Found ' . count( $posts ) . ' Posts' . PHP_EOL;

	$errors = 0;
	$success = 0;

	foreach ( $posts as $post ) {
		$categories = $post->post_category;

		// Remove the broken category.
		$categories = array_diff( $categories, [ $duplicate_category ] );

		// Add the correct one.
		$categories[] = $correct_category;

		// Remove Duplicates if any.
		$categories = array_unique( $categories );

		// Write this to WordPress.
		$result = wp_set_post_categories( $post->ID, $categories );

		if ( $result === false || $result instanceof WP_Error ) {
			echo 'Error on updating ' . $post->ID . PHP_EOL;
			$errors++;
		} else {
			echo 'Updated ' . $post->ID . PHP_EOL;
			$success++;
		}
	}

	echo 'Success: ' . $success . PHP_EOL;
	echo 'Errors: ' . $errors . PHP_EOL;


	$result = wp_delete_category( $duplicate_category );

	if ( $result === true ) {
		echo 'Deleted Category ' . $duplicate_category . PHP_EOL;
	} else {
		echo 'Failed deleting Category ' . $duplicate_category . PHP_EOL;
	}

}