php time calculation between am to pm and pm to am

if you want to calculate total time against check in and check out
then
here shown

$timestart = strtotime("09:12:31 am");
$timeend = strtotime("01:51:19 pm");
$time_difference  = $timeend - $timestart;
echo gmdate("h:i:s",$time_difference);

here is gmdate()
Format a GMT/UTC date and time and return the formatted date strings:
<?php
// Prints the day1. echo gmdate("l") . "<br>";

// Prints the day, date, month, year, time, AM or PM2. echo gmdate("l jS \of F Y h:i:s A") . "<br>";

// Prints October 3, 1975 was on a Thursday3. echo "Oct 3, 1975 was on a ".gmdate("l", mktime(0,0,0,10,3,1975)) . "<br>";

// Use a constant in the format parameter4. echo gmdate(DATE_RFC822) . "<br>";

// prints something like: 1975-10-03T00:00:00+00:005. echo gmdate(DATE_ATOM,mktime(0,0,0,10,3,1975));
?>
Results:
1. Tuesday
2. Tuesday 21st of May 2019 05:25:59 AM
3. Oct 3, 1975 was on a Friday
4. Tue, 21 May 19 05:25:59 +0000
5. 1975-10-03T04:00:00+00:00  


Required. Specifies the format of the outputted date string. The following characters can be used:

• d - The day of the month (from 01 to 31)
• D - A textual representation of a day (three letters)
• j - The day of the month without leading zeros (1 to 31)
• l (lowercase 'L') - A full textual representation of a day
• N - The ISO-8601 numeric representation of a day (1 for Monday, 7 for Sunday)
• S - The English ordinal suffix for the day of the month (2 characters st, nd, rd or th. Works well with j)
• w - A numeric representation of the day (0 for Sunday, 6 for Saturday)
• z - The day of the year (from 0 through 365)
• W - The ISO-8601 week number of year (weeks starting on Monday)
• F - A full textual representation of a month (January through December)
• m - A numeric representation of a month (from 01 to 12)
• M - A short textual representation of a month (three letters)
• n - A numeric representation of a month, without leading zeros (1 to 12)
• t - The number of days in the given month
• L - Whether it's a leap year (1 if it is a leap year, 0 otherwise)
• o - The ISO-8601 year number
• Y - A four digit representation of a year
• y - A two digit representation of a year
• a - Lowercase am or pm
• A - Uppercase AM or PM
• B - Swatch Internet time (000 to 999)
• g - 12-hour format of an hour (1 to 12)
• G - 24-hour format of an hour (0 to 23)
• h - 12-hour format of an hour (01 to 12)
• H - 24-hour format of an hour (00 to 23)
• i - Minutes with leading zeros (00 to 59)
• s - Seconds, with leading zeros (00 to 59)
• u - Microseconds (added in PHP 5.2.2)
• e - The timezone identifier (Examples: UTC, GMT, Atlantic/Azores)
• I (capital i) - Whether the date is in daylights savings time (1 if Daylight Savings Time, 0 otherwise)
• O - Difference to Greenwich time (GMT) in hours (Example: +0100)
• P - Difference to Greenwich time (GMT) in hours:minutes (added in PHP 5.1.3)
• T - Timezone abbreviations (Examples: EST, MDT)
• Z - Timezone offset in seconds. The offset for timezones west of UTC is negative (-43200 to 50400)
• c - The ISO-8601 date (e.g. 2013-05-05T16:34:42+00:00)
• r - The RFC 2822 formatted date (e.g. Fri, 12 Apr 2013 12:01:05 +0200)
• U - The seconds since the Unix Epoch (January 1 1970 00:00:00 GMT)

and the following predefined constants can also be used (available since PHP 5.1.0):

• DATE_ATOM - Atom (example: 2013-04-12T15:52:01+00:00)
• DATE_COOKIE - HTTP Cookies (example: Friday, 12-Apr-13 15:52:01 UTC)
• DATE_ISO8601 - ISO-8601 (example: 2013-04-12T15:52:01+0000)
• DATE_RFC822 - RFC 822 (example: Fri, 12 Apr 13 15:52:01 +0000)
• DATE_RFC850 - RFC 850 (example: Friday, 12-Apr-13 15:52:01 UTC)
• DATE_RFC1036 - RFC 1036 (example: Fri, 12 Apr 13 15:52:01 +0000)
• DATE_RFC1123 - RFC 1123 (example: Fri, 12 Apr 2013 15:52:01 +0000)
• DATE_RFC2822 - RFC 2822 (Fri, 12 Apr 2013 15:52:01 +0000)
• DATE_RFC3339 - Same as DATE_ATOM (since PHP 5.1.3)
• DATE_RSS - RSS (Fri, 12 Aug 2013 15:52:01 +0000)
• DATE_W3C - World Wide Web Consortium (example: 2013-04-12T15:52:01+00:00)