how can I get last week' date range in php?

Simply use

date("m/d/Y", strtotime("last week monday"));
date("m/d/Y", strtotime("last week sunday"));

It will give the date of Last week's Monday and Sunday.

You can use strtotime()

$previous_week = strtotime("-1 week +1 day");

$start_week = strtotime("last sunday midnight",$previous_week);
$end_week = strtotime("next saturday",$start_week);

$start_week = date("Y-m-d",$start_week);
$end_week = date("Y-m-d",$end_week);

echo $start_week.' '.$end_week ;

UPDATE

Changed the code to handle sunday. If the current day is sunday then - 1 week will be previous sunday and again getting previous sunday for that will go the one week back.

$previous_week = strtotime("-1 week +1 day");

In addition if we need to find the current week and next week date range we can do as

Current week -

$d = strtotime("today");
$start_week = strtotime("last sunday midnight",$d);
$end_week = strtotime("next saturday",$d);
$start = date("Y-m-d",$start_week); 
$end = date("Y-m-d",$end_week);  

Next Week -

$d = strtotime("+1 week -1 day");
$start_week = strtotime("last sunday midnight",$d);
$end_week = strtotime("next saturday",$d);
$start = date("Y-m-d",$start_week); 
$end = date("Y-m-d",$end_week); 

you need you use strtotime function for this

<center>
<?php
 function get_last_week_dates()
 {
        // how can i get the date range last week ?
        // ex: today is 2014-2-8
        // the week date range of last week should be '2014-1-26 ~ 2014-2-1'

        $startdate = "last monday";
        if (date('N') !== '1')
        { 
            // it's not Monday today
            $startdate .= " last week";
        }
        echo "<br />";
        $day = strtotime($startdate);
        echo date('r', $day);   
        echo "<br />";
        $sunday = strtotime('next monday', $day) - 1;
        echo date('r', $sunday);
    }
    get_last_week_dates();
?>
</center>