You could convert the time to the current data and then use the strtotime function to find the start of the day and simply add 24 hours to that to find the end of the day.

You could also use the remainder operator (%) to find the nearest day. For example:

$start_of_day = time() - 86400 + (time() % 86400);
$end_of_day = $start_of_day + 86400;

You can use a combination of date() and mktime():

list($y,$m,$d) = explode('-', date('Y-m-d', $ts));
$start = mktime(0,0,0,$m,$d,$y);
$end = mktime(0,0,0,$m,$d+1,$y);

mktime() is smart enough to wrap months/years when given a day outside the specified month (jan 32nd will be feb 1st, etc)

strtotime can be used to to quickly chop off the hour/minutes/seconds

$beginOfDay = strtotime("today", $timestamp);
$endOfDay   = strtotime("tomorrow", $beginOfDay) - 1;

DateTime can also be used, though requires a few extra steps to get from a long timestamp

$dtNow = new DateTime();
// Set a non-default timezone if needed
$dtNow->setTimezone(new DateTimeZone('Pacific/Chatham'));
$dtNow->setTimestamp($timestamp);


$beginOfDay = clone $dtNow;
$beginOfDay->modify('today');


$endOfDay = clone $beginOfDay;
$endOfDay->modify('tomorrow');
// adjust from the start of next day to the end of the day,
// per original question
// Decremented the second as a long timestamp rather than the
// DateTime object, due to oddities around modifying
// into skipped hours of day-lights-saving.
$endOfDateTimestamp = $endOfDay->getTimestamp();
$endOfDay->setTimestamp($endOfDateTimestamp - 1);


var_dump(
array(
'time ' => $dtNow->format('Y-m-d H:i:s e'),
'start' => $beginOfDay->format('Y-m-d H:i:s e'),
'end  ' => $endOfDay->format('Y-m-d H:i:s e'),
)
);

With the addition of extended time in PHP7, there is potential to miss a second if using $now <= $end checking with this. Using $now < $nextStart checking would avoid that gap, in addition to the oddities around subtracting seconds and daylight savings in PHP's time handling.

For anyone that have this question in the future:

Any day code

<?php
$date = "2015-04-12 09:20:00";


$midnight = strtotime("midnight", strtotime($date));
$now = strtotime($date);


$diff = $now - $midnight;
echo $diff;
?>

Current day code

<?php
$midnight = strtotime("midnight");
$now = date('U');


$diff = $now - $midnight;
echo $diff;
?>

Just DateTime

$beginOfDay = DateTime::createFromFormat('Y-m-d H:i:s', (new DateTime())->setTimestamp($timestamp)->format('Y-m-d 00:00:00'))->getTimestamp();
$endOfDay = DateTime::createFromFormat('Y-m-d H:i:s', (new DateTime())->setTimestamp($timestamp)->format('Y-m-d 23:59:59'))->getTimestamp();

First a DateTime object is created and the timestamp is set to the desired timestamp. Then the object is formatted as a string setting the hour/minute/second to the beginning or end of the day. Lastly, a new DateTime object is created from this string and the timestamp is retrieved.

Readable

$dateTimeObject = new DateTime();
$dateTimeObject->setTimestamp($timestamp);
$beginOfDayString = $dateTimeObject->format('Y-m-d 00:00:00');
$beginOfDayObject = DateTime::createFromFormat('Y-m-d H:i:s', $beginOfDayString);
$beginOfDay = $beginOfDayObject->getTimestamp();

We can get the end of the day in an alternate manner using this longer version:

$endOfDayObject = clone $beginOfDayOject(); // Cloning because add() and sub() modify the object
$endOfDayObject->add(new DateInterval('P1D'))->sub(new DateInterval('PT1S'));
$endOfDay = $endOfDayOject->getTimestamp();

Timezone

The timezone can be set as well by adding a timestamp indicator to the format such as O and specifying the timestamp after creating the DateTime object:

$beginOfDay = DateTime::createFromFormat('Y-m-d H:i:s O', (new DateTime())->setTimezone(new DateTimeZone('America/Los_Angeles'))->setTimestamp($timestamp)->format('Y-m-d 00:00:00 O'))->getTimestamp();

Flexibility of DateTime

We can also get other information such as the beginning/end of the month or the beginning/end of the hour by changing the second format specified. For month: 'Y-m-01 00:00:00' and 'Y-m-t 23:59:59'. For hour: 'Y-m-d H:00:00' and 'Y-m-d H:59:59'

Using various formats in combination with add()/sub() and DateInterval objects, we can get the beginning or end of any period, although some care will need to be taken to handle leap years correctly.

Relevant Links

From the PHP docs:

• DateTime
• date with info on the format
• DateTimeZone
• DateInterval

$start_of_day = floor (time() / 86400) * 86400;
$end_of_day = ceil (time() / 86400) * 86400;

If your need both values in the same script. It is faster to +/- 86400 seconds to one of the variables than to fire both floor and ceil. For example:

$start_of_day = floor (time() / 86400) * 86400;
$end_of_day = $start_of_day + 86400;

Today Starting date timestamp. Simple

$stamp = mktime(0, 0, 0);
echo date('m-d-Y H:i:s',$stamp);

The accepted answer unfortunately breaks due to a php bug that occurs in very specific scenarios. I'll discuss those scenarios, but first the answer using DateTime. The only difference between this and the accepted answer occurs after the // IMPORTANT line:

$dtNow = new DateTime();
// Set a non-default timezone if needed
$dtNow->setTimezone(new DateTimeZone('America/Havana'));
$dtNow->setTimestamp($timestamp);


$beginOfDay = clone $dtNow;


// Go to midnight.  ->modify('midnight') does not do this for some reason
$beginOfDay->modify('today');


// now get the beginning of the next day
$endOfDay = clone $beginOfDay;
$endOfDay->modify('tomorrow');


// IMPORTANT
// get the timestamp
$ts = $endOfDay->getTimestamp();
// subtract one from that timestamp
$tsEndOfDay = $ts - 1;


// we now have the timestamp at the end of the day. we can now use that timestamp
// to set our end of day DateTime
$endOfDay->setTimestamp($tsEndOfDay);

So you'll note that instead of using ->modify('1 second ago'); we instead get the timestamp and subtract one. The accepted answer using modify should work, but breaks because of php bug in very specific scenarios. This bug occurs in timezones that change daylight savings at midnight, on the day of the year that clocks are moved "forward". Here is an example you can use to verify that bug.

bug example code

// a time zone, Cuba, that changes their clocks forward exactly at midnight. on
// the day before they make that change. there are other time zones which do this
$timezone = 'America/Santiago';
$dateString = "2020-09-05";


echo 'the start of the day:<br>';
$dtStartOfDay = clone $dtToday;
$dtStartOfDay->modify('today');
echo $dtStartOfDay->format('Y-m-d H:i:s');
echo ', '.$dtStartOfDay->getTimestamp();


echo '<br><br>the start of the *next* day:<br>';
$dtEndOfDay = clone $dtToday;
$dtEndOfDay->modify('tomorrow');
echo $dtEndOfDay->format('Y-m-d H:i:s');
echo ', '.$dtEndOfDay->getTimestamp();


echo '<br><br>the end of the day, this is incorrect. notice that with ->modify("-1 second") the second does not decrement the timestamp by 1:<br>';
$dtEndOfDayMinusOne = clone $dtEndOfDay;
$dtEndOfDayMinusOne->modify('1 second ago');
echo $dtEndOfDayMinusOne->format('Y-m-d H:i:s');
echo ', '.$dtEndOfDayMinusOne->getTimestamp();


echo '<br><br>the end of the day, this is correct:<br>';
$dtx = clone $dtEndOfDay;
$tsx = $dtx->getTimestamp() - 1;
$dty = clone $dtEndOfDay;
$dty->setTimestamp($tsx);
echo $dty->format('Y-m-d H:i:s');
echo ', '.$tsx;

bug example code output

the start of the day:
2020-03-26 00:00:00, 1585173600


the start of the *next* day:
2020-03-27 01:00:00, 1585260000


the end of the day, this is incorrect. notice that with ->modify("1 second ago") the
second does not decrement the timestamp by 1:
2020-03-27 01:59:59, 1585263599


the end of the day, this is correct:
2020-03-26 23:59:59, 1585259999

$date = (new \DateTime())->setTimestamp(1330581600);


echo $date->modify('today')->format('Y-m-d H:i:s'); // 2012-02-29 00:00:00
echo PHP_EOL;
echo $date->modify('tomorrow - 1 second')->format('Y-m-d H:i:s'); // 2012-02-29 23:59:59

    $startOfDay = new \DateTime('tomorrow');
$startOfDay->modify('-1 day');

This works for me :)

A little late to the party, but here's another easy way to achieve what you're looking for:

$timestamp=1330581600;
$format = DATE_ATOM;


$date = (new DateTime())->setTimestamp($timestamp);
// Here's your initial date, created from the timestamp above
// 2012-03-01T06:00:00+00:00
$dateFromTimestamp = $date->format($format);


// This is the beginning of the day
// 2012-03-01T00:00:00+00:00
$startOfDay = $date->setTime(0,0);


// This is the beginning of the next day
// 2012-03-02T00:00:00+00:00
$startOfNextDay = $startOfDay->modify('+1 day');

I would personally avoid using the end of the day unless it's absolutely necessary. You can, of course, use 23:59:59 but this is not the actual end of the day (there's still 1 second left). What I do is use the start of the next day as my end boundary, for example:

$start = new DateTime('2021-11-09 00:00:00');
$end = new DateTime('2021-11-10 00:00:00');
    

if ($someDateTime >= $start && $someDateTime < $end) {
// do something
}

If I must use the end of the day, I'd go with calculating the start of the next day and then subtracting 1 microsecond from that.

$beginOfDay = (new DateTime('today', new DateTimeZone('Asia/Tehran')))->getTimestamp();
$endOfDay   = $beginOfDay + 86399;

You can set a timezone by replacing "Asia/Tehran". One day is 86400 seconds, Don't ask me why 86399, It is a whisper in my mind that says it is 86399, So I do not even want to think about its truth.