http-basic-auth-wordpress.php

<?php
function send_http_auth_headers(){
	header('WWW-Authenticate: Basic realm="Your Website Name Restricted"');
        header('HTTP/1.0 401 Unauthorized');
        echo 'Please speak to an administrator for access to the this feature.';
        exit;
}
add_action('template_redirect', 'maybe_add_http_auth_basic', 0);
function maybe_add_http_auth_basic(){
	# Add your specific URI segment (i.e. http://example.com/segment-string/)
	$uri = 'segment-string';
	if(!is_admin()){
		global $wp;
		# Look for exact match and child segments
		if($wp->request === $uri || strpos($wp->request, $uri.'/') !== false ){
			$user = HTTP_USER_NAME; # Define this in wp-config.php
			$pass = HTTP_USER_PASS; # Define this in wp-config.php
			# Check if user/pass has been entered
			if (!isset($_SERVER['PHP_AUTH_USER'])) {
				send_http_auth_headers();
			} else{
				# Check if the user/pass is correct
				if ($_SERVER['PHP_AUTH_USER'] !== $user && $_SERVER['PHP_AUTH_PW'] !== $pass) {
					send_http_auth_headers();
				}
			}
		}
	}
}