WordPress at a glance
filter is not described

protected_title_format filter-hook . WP 2.8.0

Filters the text prepended to the post title for protected posts.

The filter is only applied on the front end.

Usage

add_filter( 'protected_title_format', 'filter_function_name_202', 10, 2 );
function filter_function_name_202( $prepend, $post ){
	// filter...

	return $prepend;
}
$prepend(string)
Text displayed before the post title.
Default: 'Protected: %s'
$post(WP_Post)
Current post object.

Where the hook is called

get_the_title()
protected_title_format
wp-includes/post-template.php 130
$protected_title_format = apply_filters( 'protected_title_format', __( 'Protected: %s' ), $post );

Where the hook is used (in WP core)

wp-includes/rest-api/search/class-wp-rest-post-search-handler.php 101
add_filter( 'protected_title_format', array( $this, 'protected_title_format' ) );
wp-includes/rest-api/search/class-wp-rest-post-search-handler.php 103
remove_filter( 'protected_title_format', array( $this, 'protected_title_format' ) );
wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php 1497
add_filter( 'protected_title_format', array( $this, 'protected_title_format' ) );
wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php 1504
remove_filter( 'protected_title_format', array( $this, 'protected_title_format' ) );