方法一

/**
 * 比较两个日期相差的天数
 * $date1 开始日期  Y-m-d
 * $date2 结束日期  Y-m-d
 */
function CompareDate($date1,$date2){
	$time1 = strtotime($date1);
	$time2 = strtotime($date2);
	return ceil(($time2-$time1)/86400);
}

$date1 = '2017-03-23';
$date2 = date('Y-m-d',time());
$days = CompareDate($date1,$date2);
echo $days;

方法二

/**
* 求两个日期之间相差的天数
* (针对1970年1月1日之后，求之前可以采用泰勒公式)
* @param string $day1
* @param string $day2
* @return number
*/
function diffBetweenTwoDays ($day1, $day2){
	$second1 = strtotime($day1);
	$second2 = strtotime($day2);
	if ($second1 < $second2) {
		$tmp = $second2;
		$second2 = $second1;
		$second1 = $tmp;
	}
	return ($second1 - $second2) / 86400;
}
$day1 = "2013-07-27";
$day2 = "2013-08-04";
$diff = diffBetweenTwoDays($day1, $day2);
echo $diff."\n";

$date1=date_create("1984-01-28");
$date2=date_create("1984-10-15");
$diff=date_diff($date1,$date2);
var_dump($diff);
echo $diff->format('%R%a days');

//设置页面编码
header("Content-type:text/html;charset=utf-8");
//设置时区
date_default_timezone_set('PRC'); 
/*
 * 比较时间差
 * $from,$to开始时间，结束时间，为时间戳格式 例1540348577
 */
function get_time($from,$to){
    if($to > $from){
        $miao = $to - $from;
    }else{
        $miao = $from - $to;
    }
    $hour = floor($miao/3600);
    $minute = floor(($miao-$hour*3600)/60);
    $second = $miao - $hour * 3600 - $minute * 60 ;
    $str = '';
    if($hour > 0){
        $str .= $hour.'时';
    }
    if($minute > 0){
        $str .= $minute.'分';
    }
    if($second){
        $str .= $second.'秒';
    }
    return $str;
}

$from = strtotime('2018-10-24 00:00:00');
$to = time();
$times = get_time($from,$to);
echo '当前时间：'.date('Y-m-d h:s:i',$to).'<br>';
echo '时间差：'.$times;