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<?php 
/*
Plugin Name: Bulk Updater
Description: This is not just a plugin, it's <em>CODE</em>..
Author: 
*/
//add_action('init','KF_update_all_posts');

function KF_update_all_posts(){

    $my_posts = get_posts( array('post_type' => 'catalog', 'numberposts' => -1 ) );

    foreach ( $my_posts as $my_post ):

		$post_id = $my_post->ID;
		$value = 'P-' . $post_id;
		if(!get_field('product_sku', $post_id)){
			update_field('product_sku', $value, $post_id);
		}
		wp_update_post( $my_post );

    endforeach;
}