<?php
function getthemonth($date)
{
    $firstday = date('Y-m-01', strtotime($date));
    $nowday = date('Y-m-d', strtotime($date));
    $lastday = date('Y-m-d', strtotime("$firstday +1 month -1 day"));
    return array($firstday,$nowday,$lastday);
}
$today = date("Y-m-d");
$day=getthemonth($today);
echo "当月的第一天: ".$day[0]." 当天日期: ".$day[1]." 当月的最后一天: ".$day[2];
echo "<br/>";
if($day[0] == $day[1]){
    echo '今天的日期是当月的第一天';
}else{
    $num = explode('-',$day[1]);
    echo '今天的日期不是当月的第一天,而是第'.$num[2].'天';
}
?>