result.txt

$ php some-bench.php 
strtotime: 0.15609812736511
datetime: 0.31521701812744
datetime without instanciation: 0.11794400215149
custom: 0.70522284507751

timestamp-bench.php

<?php

// See https://en.code-bude.net/2013/12/19/benchmark-strtotime-vs-datetime-vs-gettimestamp-in-php/

$start = microtime(true);
for ($i = 0; $i < 100000; $i ++) {
  $a=strtotime("2012-05-28");
}

echo "strtotime: ", (microtime(true) - $start), "\n";


$start = microtime(true);
for ($i = 0; $i < 100000; $i ++) {
  $date=new DateTime("2012-05-28");
  $a = $date->getTimestamp();
}

echo "datetime: ", (microtime(true) - $start), "\n";

$date=new DateTime("2012-05-28");
$start = microtime(true);
for ($i = 0; $i < 100000; $i ++) {
  $a = $date->getTimestamp();
}

echo "datetime without instanciation: ", (microtime(true) - $start), "\n";





$start = microtime(true);
for ($i = 0; $i < 100000; $i ++) {
  $a = getTimestamp("2012-05-28");
}

echo "custom: ", (microtime(true) - $start) , "\n";





function getTimestamp($timeStr)
{
    $daysInMonth = array(0,31,59,90,120,151,181,212,243,273,304,334);
    $dateArr = explode("-", $timeStr);
 
    //Add years to the day of the year entered
    $days = ($dateArr[0]-1970) * 365;
 
    //Add leap years, days
    for ($i = 1972; $i <= $dateArr[0]; $i+=4)
    {
        if (($i%4==0 && $i%100 != 0) || $i%400==0)
            $days++;
    }
 
    //Days until 31.03. of the year
    $daysTimeswitch = $days + 90;
    //Determine day of the week using MOD 3 . (+3 because 01.01.1970 = Thursday)
    //Pull off day of the week from 31. = last Sunday of the month => Daylight saving time
    $daySummertime = 31 - ((($daysTimeswitch%7) + 3)%7);
 
    $daysTimeswitch = $days + 304;
    $dayWintertime = 31 - ((($daysTimeswitch%7) + 3)%7);
 
    if (($dateArr[1] > 3 && $dateArr[1] < 10)
        || ($dateArr[1] == 3 && $dateArr[2] > $daySummertime)
        || ($dateArr[1] == 10 && $dateArr[2] <= $dayWintertime))
    {
         $summerWinterTime = 3600;
    }
    else{
         $summerWinterTime = 0;
    }
 
    //Pull off leap year day
    $days -= (($dateArr[0]%4==0 &&  $dateArr[0]%100 != 0) ||  $dateArr[0]%400==0) ? ($dateArr[1] > 2) ? 0 : 1 : 0;
 
    //Add days to months of the current year
    $days += $daysInMonth[$dateArr[1]-1];
 
    //Add days of the current year
    $days += $dateArr[2] - 1;
 
    //Pull off 3600 seconds for time shift. (Unix timestamp / UTC)
    return $days * 86400 - 3600 - $summerWinterTime;
}