To compute the correct Easter date for Eastern Orthodox Churches I made a function based on the Meeus Julian algorithm:

<?php
function orthodox_eastern($year) {
    $a = $year % 4;
    $b = $year % 7;
    $c = $year % 19;
    $d = (19 * $c + 15) % 30;
    $e = (2 * $a + 4 * $b - $d + 34) % 7;
    $month = floor(($d + $e + 114) / 31);
    $day = (($d + $e + 114) % 31) + 1;
    
    $de = mktime(0, 0, 0, $month, $day + 13, $year);
    
    return $de;
}
?>

Thank you, @Maxie, for algorythm for computing Orthodox Easter date.
It can be improved though. You added 13 days in order to map Julian calendar to Gregorian.  But 13 days is not a constant. It's an accumulated error fixed in Gregorian and should be calculated with this formula: (int)($year / 100) - (int)($year / 400) - 2

I recently had to write a function that allows me to know if today is a holiday.

And in France, we have some holidays which depends on the easter date. Maybe this will be helpful to someone.

Just modify in the $holidays array the actual holidays dates of your country.

<?php
/**
 * This function returns an array of timestamp corresponding to french holidays
 */
protected static function getHolidays($year = null)
{
  if ($year === null)
  {
    $year = intval(date('Y'));
  }
    
  $easterDate  = easter_date($year);
  $easterDay   = date('j', $easterDate);
  $easterMonth = date('n', $easterDate);
  $easterYear   = date('Y', $easterDate);

  $holidays = array(
    // These days have a fixed date
    mktime(0, 0, 0, 1,  1,  $year),  // 1er janvier
    mktime(0, 0, 0, 5,  1,  $year),  // Fête du travail
    mktime(0, 0, 0, 5,  8,  $year),  // Victoire des alliés
    mktime(0, 0, 0, 7,  14, $year),  // Fête nationale
    mktime(0, 0, 0, 8,  15, $year),  // Assomption
    mktime(0, 0, 0, 11, 1,  $year),  // Toussaint
    mktime(0, 0, 0, 11, 11, $year),  // Armistice
    mktime(0, 0, 0, 12, 25, $year),  // Noel

    // These days have a date depending on easter
    mktime(0, 0, 0, $easterMonth, $easterDay + 2,  $easterYear),
    mktime(0, 0, 0, $easterMonth, $easterDay + 40, $easterYear),
    mktime(0, 0, 0, $easterMonth, $easterDay + 50, $easterYear),
  );

  sort($holidays);
  
  return $holidays;
}
?>

I found a problem with holidays timestamp computation and daylight saving time.
An article about it at http://goo.gl/76t31 (in french only, sorry).

In summary, this year (2013) easter begins before adding an hour for daylight saving time (occured sunday at 3:00). It means that if you do $easter + X, where x is a number of seconds equivalent to one day, 39 days or 50 days, the result is not equals to a midnight timestamp...

Here a function to check if a midnight timestamp is equals to an holiday :

function isHoliday( $ts ) {
// Licence : Creative Commons (BY)
// By Webpulser - http://goo.gl/76t31
  $fixed_holidays = array( ’01-01′, ’01-05′, ’08-05′, ’14-07′, ’15-08′, ’11-11′, ’25-12′ );
  $format = ‘d-m’;

 $dm = date($format, $ts);
  if ( in_array($dm, $fixed_holidays) ) return true;

 $easter = easter_date( date(‘Y’, $ts) );
  if ( date($format, $easter +   86400) == $dm ) return true;
  if ( date($format, $easter + 3369600) == $dm ) return true;
  if ( date($format, $easter + 4320000) == $dm ) return true;

 return false;
}

feel free to use / modify.

The algorithm from Bigtree is correct if you add some (int) cast
<?php 
   function easter_date ($Year) { 
   
       /* 
       G is the Golden Number-1 
       H is 23-Epact (modulo 30) 
       I is the number of days from 21 March to the Paschal full moon 
       J is the weekday for the Paschal full moon (0=Sunday, 
         1=Monday, etc.) 
       L is the number of days from 21 March to the Sunday on or before 
         the Paschal full moon (a number between -6 and 28) 
       */ 
       

         $G = $Year % 19; 
         $C = (int)($Year / 100); 
         $H = (int)($C - (int)($C / 4) - (int)((8*$C+13) / 25) + 19*$G + 15) % 30; 
         $I = (int)$H - (int)($H / 28)*(1 - (int)($H / 28)*(int)(29 / ($H + 1))*((int)(21 - $G) / 11)); 
         $J = ($Year + (int)($Year/4) + $I + 2 - $C + (int)($C/4)) % 7; 
         $L = $I - $J; 
         $m = 3 + (int)(($L + 40) / 44); 
         $d = $L + 28 - 31 * ((int)($m / 4)); 
         $y = $Year; 
         $E = mktime(0,0,0, $m, $d, $y); 

         return $E; 

   } 
?>

Hey, recently I needed a function to get realization dates in online shop, so here it is (ready to go for polish users, please adjust your dates for any other country):

<?php
function getWorkday($date1,$workDays) {
        $workDays = (int)$workDays;
        if ($workDays <= 0)
            return null;

        $date1=strtotime('-1 day',strtotime($date1));

        $lastYear = null;
        $hol=array('01-01','01-06','05-01','05-03','08-15','11-01','11-11','12-25','12-26');  //array of month-date of static holidays (these are from Poland)
        $i = 0;
        while ($i<=$workDays) {
            $year = date('Y', $date1);
            if ($year !== $lastYear){
                $lastYear = $year;
                $easter = date('m-d', easter_date($year));
                $date = strtotime($year . '-' . $easter); // easter
                $easterSec = date('m-d', strtotime('+1 day', $date)); // easter monday
                $greens = date('m-d', strtotime('+49 days', $date)); // zielone swiatki
                $cc = date('m-d', strtotime('+60 days', $date)); // boze cialo
                $hol[] = $easter;
                $hol[] = $easterSec;
                $hol[] = $greens;
                $hol[] = $cc;
            }
            $weekDay=date('w',$date1);
            if (!($weekDay==0 || $weekDay==6 || in_array(date('m-d',$date1),$hol)))
                $i++;

            $date1=strtotime('+1 day',$date1);
        }
        return date('Y-m-d',$date1);
    }
?>