PHP code to implement the last day of the previous month and the last day of a certain month

<?php
    //显示上个月的最后一天  
    function G_L_M_Lastday($dateStr=''){  
    $time = time();  
    if($dateStr !='')  
       $time = strtotime($dateStr);   
        return date('Y-m-t', strtotime('last month', $time));  
    }  
      
    echo G_L_M_Lastday();    // 默认显示当前日期的上月最后一天  
    echo "<br>";  
    echo G_L_M_Lastday('2010-3-3'); // 显示给定日期的上月最后一天  
    echo "<hr>"; 
   //by http://bbs.it-home.org
?>

<?php
    //某个月的最后一天  
    $lastday = mktime(0, 0, 0, 3, 0, 2011);  
    echo strftime("Last day in Feb 2011 is: %d", $lastday);//2月的最后一天  
    $lastday1 = mktime(0, 0, 0, 4, -31, 2000);  
    echo strftime("Last day in Feb 2000 is: %d", $lastday1);//2月的最后一天  
?>

<?php
    $time = strtotime('last month');  
    echo date('Y-m-t', $time); //上个月的最后一天  
    echo date('Y-m-01', $time); //上个月的第一天
?>