plugin.php

<?php
/*
 * Plugin Name: Foo Bar
 */

add_action( 'save_post', 'my_save_post' );
function my_save_post( $post_id ) {
    if ( wp_is_post_revision( $post_id ) )
        return;

    $post = get_post( $post_id );
    $post->post_content = str_replace( 'foo', 'bar', $post->post_content );

    remove_action( 'save_post', 'my_save_post' ); // избегаем infinite loop
    wp_update_post( $post );
    add_action( 'save_post', 'my_save_post' ); // добавляем хук обратно
}