Chamando CSS apenas quando for mobile

<?php
    $iphone = strpos($_SERVER['HTTP_USER_AGENT'],"iPhone");
    $android = strpos($_SERVER['HTTP_USER_AGENT'],"Android");
    $palmpre = strpos($_SERVER['HTTP_USER_AGENT'],"webOS");
    $berry = strpos($_SERVER['HTTP_USER_AGENT'],"BlackBerry");
    $ipod = strpos($_SERVER['HTTP_USER_AGENT'],"iPod");

    // Verificando se é mobile
    if ($iphone || $android || $palmpre || $ipod || $berry == true) {
        echo "<link rel="stylesheet" type="text/css" href="bootstrap/css/bootstrap-theme.min.css">
                    <link rel="stylesheet" type="text/css" href="bootstrap/css/bootstrap.min.css">
 ";
    } else {
        echo "<script type="text/javascript" src="bootstrap/js/bootstrap.min.js">
              <script type="text/javascript" src="bootstrap/js/npm.js"></script>
              <link rel="stylesheet" type="text/css" href="css/principal.css">";
    }
?>