How to filter the response for get_post_thumbnail_id()

filter-post-thumbnail-id.php

<?php
/**
 * How to filter the value that would be returned by get_post_thumbnail_id()
 */

add_filter( 'get_post_metadata', function ( $value, $post_id, $meta_key, $single ) {
	// We want to pass the actual _thumbnail_id into the filter, so requires recursion
	static $is_recursing = false;

	// Only filter if we're not recursing and if it is a post thumbnail ID
	if ( ! $is_recursing && $meta_key === '_thumbnail_id' ) {
		$is_recursing = true; // prevent this conditional when get_post_thumbnail_id() is called
		$value = get_post_thumbnail_id( $post_id );
		$is_recursing = false;
		$value = apply_filters( 'post_thumbnail_id', $value, $post_id ); // yay!
		if ( ! $single ) {
			$value = array( $value );
		}
	}
	return $value;
}, 10, 4);

wp-meta.php

<?php
// Copied from core for reference

/**
 * Retrieve metadata for the specified object.
 *
 * @since 2.9.0
 *
 * @param string $meta_type Type of object metadata is for (e.g., comment, post, or user)
 * @param int $object_id ID of the object metadata is for
 * @param string $meta_key Optional. Metadata key. If not specified, retrieve all metadata for
 *     	the specified object.
 * @param bool $single Optional, default is false. If true, return only the first value of the
 * 		specified meta_key. This parameter has no effect if meta_key is not specified.
 * @return string|array Single metadata value, or array of values
 */
function get_metadata($meta_type, $object_id, $meta_key = '', $single = false) {
	if ( !$meta_type )
		return false;

	if ( !$object_id = absint($object_id) )
		return false;

	$check = apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, $single );
	if ( null !== $check ) {
		if ( $single && is_array( $check ) )
			return $check[0];
		else
			return $check;
	}

	$meta_cache = wp_cache_get($object_id, $meta_type . '_meta');

	if ( !$meta_cache ) {
		$meta_cache = update_meta_cache( $meta_type, array( $object_id ) );
		$meta_cache = $meta_cache[$object_id];
	}

	if ( !$meta_key )
		return $meta_cache;

	if ( isset($meta_cache[$meta_key]) ) {
		if ( $single )
			return maybe_unserialize( $meta_cache[$meta_key][0] );
		else
			return array_map('maybe_unserialize', $meta_cache[$meta_key]);
	}

	if ($single)
		return '';
	else
		return array();
}

wp-post-thumbnail-template.php

<?php
// Copied from core for reference

/**
 * Retrieve Post Thumbnail ID.
 *
 * @since 2.9.0
 *
 * @param int $post_id Optional. Post ID.
 * @return int
 */
function get_post_thumbnail_id( $post_id = null ) {
	$post_id = ( null === $post_id ) ? get_the_ID() : $post_id;
	return get_post_meta( $post_id, '_thumbnail_id', true );
}

wp-post.php

<?php
// Copied from core for reference

/**
 * Retrieve post meta field for a post.
 *
 * @since 1.5.0
 * @uses $wpdb
 * @link http://codex.wordpress.org/Function_Reference/get_post_meta
 *
 * @param int $post_id Post ID.
 * @param string $key Optional. The meta key to retrieve. By default, returns data for all keys.
 * @param bool $single Whether to return a single value.
 * @return mixed Will be an array if $single is false. Will be value of meta data field if $single
 *  is true.
 */
function get_post_meta($post_id, $key = '', $single = false) {
	return get_metadata('post', $post_id, $key, $single);
}

Really cool. Seemed really basic to filter thumbnail ID function until I saw it's connected to the function that gets all post meta. Luckily I stumbled upon your solution. Works like a charm. Many thanks sir! 🍻

Just a heads up that the post_thumbnail_id filter is now available in core since WP 5.9 - yay!