WordPress Simple 301 Redirect.

functions.php

function http_host_301_redirect() {
    $site_host = parse_url( get_bloginfo('url', 'raw' ) );
    $site_host = $site_host['host'];
    $http_host = $_SERVER["HTTP_HOST"];
    $redirect  = ( empty( $_SERVER["HTTPS"] ) ? "http://" : "https://" ) . $site_host . $_SERVER["REQUEST_URI"];

    if ( $site_host === $http_host || !wp_http_validate_url( $redirect ) )
        return;

    wp_redirect( $redirect, 301 );
}
add_action( 'template_redirect', 'http_host_301_redirect', 0 );

Can be redirected.

http://old.example.com/hoge -> http://new.example.com/hoge

Can not be redirected.

http://old.example.com/hoge -> http://new.example.com/fuga