WordPress at a glance
function is not described

_wp_post_thumbnail_class_filter_add() WP 2.9.0

Adds '_wp_post_thumbnail_class_filter' callback to the 'wp_get_attachment_image_attributes' filter hook. Internal use only.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

No Hooks.

Return

null. Null. Nothing.

Usage

_wp_post_thumbnail_class_filter_add( $attr );
$attr(string[]) (required)
Array of thumbnail attributes including src, class, alt, title, keyed by attribute name.

Changelog

Since 2.9.0 Introduced.

Code of _wp_post_thumbnail_class_filter_add() WP 5.7

<?php
function _wp_post_thumbnail_class_filter_add( $attr ) {
	add_filter( 'wp_get_attachment_image_attributes', '_wp_post_thumbnail_class_filter' );
}