How to Hook in WordPress Metadata | http://www.ibenic.com/hook-wordpress-metadata

add.php

<?php

// Should this data be added?
apply_filters( "add_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $unique );

// Do something before we add the data
do_action( "add_{$meta_type}_meta", $object_id, $meta_key, $_meta_value );

// Do Something after the data has been added	
do_action( "added_{$meta_type}_meta", $mid, $object_id, $meta_key, $_meta_value );

add_user_before.php

<?php

add_action( "add_user_meta", "notify_on_nickname", 20, 3 );
add_action( "added_user_meta", "change_not_allowed_nickname", 20, 4 );

/**
* Let's notify our admin on nickname change
* @param int    $object_id  Object ID.
* @param string $meta_key   Meta key.
* @param mixed  $meta_value Meta value.
*/
function notify_on_nickname( $object_id, $meta_key, $_meta_value ) {
  
   if( 'nickname' == $meta_key && $_meta_value == 'admin' ) {
      $user = get_userdata( $object_id );
      wp_mail( get_option('admin_email'), 'A user trying to add admin nickname', 'The user ' . $user->user_login . ' tried to change the nickname to "admin"' );
   }
}

/**
* Let's change the nickname
* @param int    $mid        Meta ID
* @param int    $object_id  Object ID.
* @param string $meta_key   Meta key.
* @param mixed  $meta_value Meta value.
*/
function change_not_allowed_nickname( $mid, $object_id, $meta_key, $_meta_value ) {
  
   if( 'nickname' == $meta_key && $_meta_value == 'admin' ) {
      update_user_meta( $object_id, $meta_key, 'adminwannabe' );
   }
}

add_user_check.php

<?php

add_filter( 'add_user_metadata', 'check_to_add_user_data', 20, 5 );

/**
* Check if the current user can have such data
* @param null|bool $check      Whether to allow adding metadata for the given type.
* @param int       $object_id  Object ID.
* @param string    $meta_key   Meta key.
* @param mixed     $meta_value Meta value. Must be serializable if non-scalar.
* @param bool      $unique     Whether the specified meta key should be unique
*                              for the object. Optional. Default false.
*/
function check_to_add_user_data( $check, $object_id, $meta_key, $meta_value, $unique ) {
  // If $check != null, then this won't be added
  if( ! current_user_can( 'have_this_saved' ) && get_current_user_id() === $object_id )  {
     $check = false;
  }
  
  return $check;
}

delete.php

<?php

// Should this data be added?
apply_filters( "delete_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $delete_all );

// Do something before we add the data
do_action( "delete_{$meta_type}_meta", $meta_ids, $object_id, $meta_key, $_meta_value );

// An action added for post meta in the core
do_action( 'delete_postmeta', $meta_ids );

// Do Something after the data has been added	
do_action( "deleted_{$meta_type}_meta", $meta_ids, $object_id, $meta_key, $_meta_value );

// An action added for post meta in the core
do_action( 'deleted_postmeta', $meta_ids );

delete_filter.php

<?php

add_filter( 'delete_user_metadata', 'block_deleting_all_nicknames', 20, 5 );

/**
* We don't want to delete all user nicknames at once
*/
function block_deleting_all_nicknames( $check, $object_id, $meta_key, $meta_value, $delete_all ) {
  if( 'nickname' === $meta_key && $delete_all ) {
    return false;
  }
  
  return $check;
}

get.php

<?php

apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, $single );

get_meta.php

<?php

add_filter( 'get_user_metadata', 'retrieve_user_nickname', 20, 4 );

/**
* Return random nickname
*/
function retrieve_user_nickname( $check, $object_id, $meta_key, $single ) {
  if( 'nickname' == $meta_key ) {
    return get_random_nickname();
  }
  return $check;
}

infinite_update.php

<?php

/**
* Example of an infinite loop. AVOID 
*/
add_action( 'updated_user_meta', 'change_user_nickname', 20, 4 );

function change_user_nickname( $meta_id, $object_id, $meta_key, $_meta_value ) {
    // Possible Infinite Loop!  
    update_user_meta( $object_id, $meta_key, 'new_value_something' );
  
}

update.php

<?php

// Should this data be added?
apply_filters( "update_{$meta_type}_metadata", null, $object_id, $meta_key, $meta_value, $prev_value );

// Do something before we add the data
do_action( "update_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

// An action added for post meta in the core
do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );

// Do Something after the data has been added	
do_action( "updated_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

// An action added for post meta in the core
do_action( 'updated_postmeta', $meta_id, $object_id, $meta_key, $meta_value );