画面サイズもしくはスマホの場合のCSS切り替え方法

<link href="/css/small.css" rel="stylesheet" media="only screen and (min-width: 0px) and (max-width: 768px)">

@media screen and (min-width:961px) {
/*pc*/
}
  
@media only screen and (min-width:376px) and (max-width:960px) {
/*tablet*/
}
 
@media screen and (max-width:375px) {
/*スマホ*/
}

<?php
$ua = $_SERVER['HTTP_USER_AGENT'];
$browser = ((strpos($ua,'iPhone')!==false)||(strpos($ua,'iPod')!==false)||(strpos($ua,'Android')!==false));
 ?>

<?php if($browser=='sp') { ?>
<link href="/css/small.css" rel="stylesheet">
<?php } ?>