WordPress at a glance
filter is not described

wp_embed_handler_youtube filter-hook . WP 4.0.0

Filters the YoutTube embed output.

Usage

add_filter( 'wp_embed_handler_youtube', 'filter_function_name_538', 10, 4 );
function filter_function_name_538( $embed, $attr, $url, $rawattr ){
	// filter...

	return $embed;
}
$embed(string)
YouTube embed output.
$attr(array)
An array of embed attributes.
$url(string)
The original URL that was matched by the regex.
$rawattr(array)
The original unmodified attributes.

Where the hook is called

wp_embed_handler_youtube()
wp-includes/embed.php 243
return apply_filters( 'wp_embed_handler_youtube', $embed, $attr, $url, $rawattr );

Where the hook is used (in WP core)

Does not used.