判断：
6点到9点就调用 echo "/css/css1.css"
10点到14点就调用 echo "/css/css2.css"
15点到17点就调用 echo "/css/css3.css"
18点到21点就调用 echo "/css/css4.css"
22点到次日点就调用 echo "/css/css4.css"
?>
date_default_timezone_set('PRC');
$hour = date('H',time());
if($hour>6 && $hour<9){
$css = "/css/css1.css";}
elseif($hour>10 && $hour<14){
$css = "/css/css2.css";}
elseif($hour>15 && $hour<17){
$css = "/css/css3.css";}
elseif($hour>18 && $hour<21){
$css = "/css/css4.css";}
else{$css = "/css/css5.css";}
echo "现在是{$hour}点，调用{$css}";
echo "";
?>