WordPress at a glance
filter is not described

http_headers_useragent filter-hook . WP 1.0

This filter is documented in wp-includes/class-http.php

Usage

add_filter( 'http_headers_useragent', 'filter_function_name_2376', 10, 2 );
function filter_function_name_2376( $string, $pagelinkedfrom ){
	// filter...

	return $string;
}
$string
-
$pagelinkedfrom
-

Where the hook is called

wp_xmlrpc_server::pingback_ping()
http_headers_useragent
WP_Http::request()
http_headers_useragent
wp-includes/class-wp-xmlrpc-server.php 6862
$user_agent = apply_filters( 'http_headers_useragent', 'WordPress/' . get_bloginfo( 'version' ) . '; ' . get_bloginfo( 'url' ), $pagelinkedfrom );
wp-includes/class-http.php 191
'user-agent'          => apply_filters( 'http_headers_useragent', 'WordPress/' . get_bloginfo( 'version' ) . '; ' . get_bloginfo( 'url' ), $url ),

Where in WP core the hook is used WordPress

Usage not found.