ディレクトリを作成したい時なんかにmicro秒を使いたい時があって
どうやったかのメモ

$time_ar = explode(" ",microtime());
echo "UnixTime:{$time_ar[1]}<br>\n";
echo "MicroTime:{$time_ar[0]}<br>\n";
echo "U+M:".$time_ar[0] + $time_ar[1];

普通にこの様に足してしまうと

UnixTime:1457315839
MicroTime:0.57940100
U+M:1457315839.5794

と出たり

UnixTime:1457315839
MicroTime:0.57900100
U+M:1457315839.579

と出たり

で、桁数が安定せず見た目が良くないので

$time_ar = explode(" ",microtime());
echo "UnixTime:{$time_ar[1]}<br>\n";
echo "MicroTime:{$time_ar[0]}<br>\n";
echo "U+M:".str_pad($time_ar[0] + $time_ar[1],15,'0',STR_PAD_RIGHT);

って感じでやると

UnixTime:1457315839
MicroTime:0.57900100
U+M:1457315839.5790

こうなって桁が揃って見栄えがいい