$t = time();
$t1 = mktime(0,0,0,date("m",$t),date("d",$t),date("Y",$t));
$t2 = mktime(0,0,0,date("m",$t),1,date("Y",$t));
$t3 = mktime(0,0,0,date("m",$t)-1,1,date("Y",$t));
$t4 = mktime(0,0,0,1,1,date("Y",$t));
$e1 = mktime(23,59,59,date("m",$t),date("d",$t),date("Y",$t));
$e2 = mktime(23,59,59,date("m",$t),date("t"),date("Y",$t));
$e3 = mktime(23,59,59,date("m",$t)-1,date("t",$t3),date("Y",$t));
$e4 = mktime(23,59,59,12,31,date("Y",$t));
//测试
echo date("当前 Y-m-d H:i:s",$t)." $t<br>";
echo date("今天起点 Y-m-d H:i:s",$t1)." $t1<br>";
echo date("今月起点 Y-m-d H:i:s",$t2)." $t2<br>";
echo date("上月起点 Y-m-d H:i:s",$t3)." $t3<br>";
echo date("今年起点 Y-m-d H:i:s",$t4)." $t4<br>";
//测试
echo date("今天终点 Y-m-d H:i:s",$e1)." $e1<br>";
echo date("今月终点 Y-m-d H:i:s",$e2)." $e2<br>";
echo date("上月终点 Y-m-d H:i:s",$e3)." $e3<br>";
echo date("今年终点 Y-m-d H:i:s",$e4)." $e4<br>";