Given a Unix timestamp, how to get beginning and end of that day?

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.

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