How to catch and modify custom field values when a page is updated

Question

I'm trying to find how to intercept the values in some custom fields on a page when they are being saved. I have searched and found out how to add a hook to the functions file to get the standard content fields, but so far haven't worked out how to do the same for custom fields. I'm using ACF Pro with flexible content fields, so the actual values come through as mutli-dimensional arrays.

The code that I am using is at the bottom. This code successfully modifies the standard 'post_content' field before saving. Basically I want to do the same for various custom fields. Any ideas?

function save_my_post( $content ) {
  global $post;
  if( isset($post) && get_post_type( $post->ID ) == 'post' ){
    $content['post_content'] = function_to_manipulate_the_content();
  }
  return $content;
}
add_filter( 'wp_insert_post_data', 'save_my_post' );

Answer 1

It all depends on how your "custom fields" are saved. If you store the values as post meta then you would have to call the update_post_meta function to update them on wp_insert_post_data. In the example below Im setting the post meta "my_meta_key" with the string value "my_meta_value".

function save_my_post( $content ) {
    global $post;
    if( isset($post) && get_post_type( $post->ID ) == 'post' ){
        update_post_meta( $post->ID, 'my_meta_key', 'my_meta_value' );
    }
    return $content;
}
add_filter( 'wp_insert_post_data', 'save_my_post' );