A couple of algoritms for calculating the date of Easter

Computus.php

<?php

class Computus
{

    public static function gauss(int $year)
    {
        $a = $year % 19;
        $b = $year % 4;
        $c = $year % 7;
        $k = floor($year / 100);
        $p = floor((13 + 8 * $k) / 25);
        $q = floor($k / 4);
        $M = (15 - $p + $k - $q) % 30;
        $N = (4 + $k - $q) % 7;
        $d = (19 * $a + $M) % 30;
        $e = (2 * $b + 4 * $c + 6 * $d + $N) % 7;

        $day = 22 + $d + $e;
        $month = '03';

        if ($day > 31) {
            $day = $d + $e - 9;
            $month = '04';
        }

        if ($d === 29 && $e === 6) {
            $day = 19;
        }

        if ($d === 28 && $e === 6 && (11 * $M + 11) % 30 < 19) {
            $day = 18;
        }

        return sprintf('%s-%s-%02d', $year, $month, $day);
    }

    public static function anonymous(int $year)
    {
        $a = $year % 19;
        $b = floor($year / 100);
        $c = $year % 100;
        $d = floor($b / 4);
        $e = $b % 4;
        $f = floor(($b + 8) / 25);
        $g = floor(($b - $f + 1) / 3);
        $h = (19 * $a + $b - $d - $g + 15) % 30;
        $i = floor($c / 4);
        $k = $c % 4;
        $l = (32 + 2 * $e + 2 * $i - $h - $k) % 7;
        $m = floor(($a + 11 * $h + 22 * $l) / 451);

        $month = floor(($h + $l - 7 * $m + 114) / 31);
        $day = (($h + $l - 7 * $m + 114) % 31) + 1;

        return sprintf('%s-%02s-%02d', $year, $month, $day);
    }

    /**
     * @param int $year
     *
     * @see https://web.archive.org/web/20150227133210/http://www.merlyn.demon.co.uk/estralgs.txt
     * @return string
     */
    public static function petrofsky(int $year)
    {
        $a = $year / 100 | 0;
        $b = $a / 4 | 0;
        $c = ($a + 1258) * 2267;
        $d = $year % 19;
        $e = (19 * $d + (($c / 25 | 0) - $b)) % 30;
        $f = $e - ((($e * 11 + $d) / 319) | 0);

        $g = 120 + $f - ($f + $year + (($year / 4) | 0) + $b + $c) % 7;

        $month = $g / 31 | 0;
        $day = $g % 31 + 1;

        return sprintf('%s-%02s-%02d', $year, $month, $day);
    }
}

ComputusTest.php

<?php

use PHPUnit\Framework\TestCase;

class ComputusTest extends TestCase
{
    /**
     * @test
     * @dataProvider easter
     */
    public function gauss($date)
    {
        $year = explode('-', $date)[0];
        $this->assertEquals($date, Computus::gauss($year));
    }

    /**
     * @test
     * @dataProvider easter
     */
    public function anonymous($date)
    {
        $year = explode('-', $date)[0];
        $this->assertEquals($date, Computus::anonymous($year));
    }

    /**
     * @test
     * @dataProvider easter
     */
    public function petrofsky($date)
    {
        $year = explode('-', $date)[0];
        $this->assertEquals($date, Computus::petrofsky($year));
    }

    public function easter()
    {
        return [
            ["1800-04-13"],
            ["1801-04-05"],
            ["1802-04-18"],
            ["1803-04-10"],
            ["1804-04-01"],
            ["1805-04-14"],
            ["1806-04-06"],
            ["1807-03-29"],
            ["1808-04-17"],
            ["1809-04-02"],
            ["1810-04-22"],
            ["1811-04-14"],
            ["1812-03-29"],
            ["1813-04-18"],
            ["1814-04-10"],
            ["1815-03-26"],
            ["1816-04-14"],
            ["1817-04-06"],
            ["1818-03-22"],
            ["1819-04-11"],
            ["1820-04-02"],
            ["1821-04-22"],
            ["1822-04-07"],
            ["1823-03-30"],
            ["1824-04-18"],
            ["1825-04-03"],
            ["1826-03-26"],
            ["1827-04-15"],
            ["1828-04-06"],
            ["1829-04-19"],
            ["1830-04-11"],
            ["1831-04-03"],
            ["1832-04-22"],
            ["1833-04-07"],
            ["1834-03-30"],
            ["1835-04-19"],
            ["1836-04-03"],
            ["1837-03-26"],
            ["1838-04-15"],
            ["1839-03-31"],
            ["1840-04-19"],
            ["1841-04-11"],
            ["1842-03-27"],
            ["1843-04-16"],
            ["1844-04-07"],
            ["1845-03-23"],
            ["1846-04-12"],
            ["1847-04-04"],
            ["1848-04-23"],
            ["1849-04-08"],
            ["1850-03-31"],
            ["1851-04-20"],
            ["1852-04-11"],
            ["1853-03-27"],
            ["1854-04-16"],
            ["1855-04-08"],
            ["1856-03-23"],
            ["1857-04-12"],
            ["1858-04-04"],
            ["1859-04-24"],
            ["1860-04-08"],
            ["1861-03-31"],
            ["1862-04-20"],
            ["1863-04-05"],
            ["1864-03-27"],
            ["1865-04-16"],
            ["1866-04-01"],
            ["1867-04-21"],
            ["1868-04-12"],
            ["1869-03-28"],
            ["1870-04-17"],
            ["1871-04-09"],
            ["1872-03-31"],
            ["1873-04-13"],
            ["1874-04-05"],
            ["1875-03-28"],
            ["1876-04-16"],
            ["1877-04-01"],
            ["1878-04-21"],
            ["1879-04-13"],
            ["1880-03-28"],
            ["1881-04-17"],
            ["1882-04-09"],
            ["1883-03-25"],
            ["1884-04-13"],
            ["1885-04-05"],
            ["1886-04-25"],
            ["1887-04-10"],
            ["1888-04-01"],
            ["1889-04-21"],
            ["1890-04-06"],
            ["1891-03-29"],
            ["1892-04-17"],
            ["1893-04-02"],
            ["1894-03-25"],
            ["1895-04-14"],
            ["1896-04-05"],
            ["1897-04-18"],
            ["1898-04-10"],
            ["1899-04-02"],
            ["1900-04-15"],
            ["1901-04-07"],
            ["1902-03-30"],
            ["1903-04-12"],
            ["1904-04-03"],
            ["1905-04-23"],
            ["1906-04-15"],
            ["1907-03-31"],
            ["1908-04-19"],
            ["1909-04-11"],
            ["1910-03-27"],
            ["1911-04-16"],
            ["1912-04-07"],
            ["1913-03-23"],
            ["1914-04-12"],
            ["1915-04-04"],
            ["1916-04-23"],
            ["1917-04-08"],
            ["1918-03-31"],
            ["1919-04-20"],
            ["1920-04-04"],
            ["1921-03-27"],
            ["1922-04-16"],
            ["1923-04-01"],
            ["1924-04-20"],
            ["1925-04-12"],
            ["1926-04-04"],
            ["1927-04-17"],
            ["1928-04-08"],
            ["1929-03-31"],
            ["1930-04-20"],
            ["1931-04-05"],
            ["1932-03-27"],
            ["1933-04-16"],
            ["1934-04-01"],
            ["1935-04-21"],
            ["1936-04-12"],
            ["1937-03-28"],
            ["1938-04-17"],
            ["1939-04-09"],
            ["1940-03-24"],
            ["1941-04-13"],
            ["1942-04-05"],
            ["1943-04-25"],
            ["1944-04-09"],
            ["1945-04-01"],
            ["1946-04-21"],
            ["1947-04-06"],
            ["1948-03-28"],
            ["1949-04-17"],
            ["1950-04-09"],
            ["1951-03-25"],
            ["1952-04-13"],
            ["1953-04-05"],
            ["1954-04-18"],
            ["1955-04-10"],
            ["1956-04-01"],
            ["1957-04-21"],
            ["1958-04-06"],
            ["1959-03-29"],
            ["1960-04-17"],
            ["1961-04-02"],
            ["1962-04-22"],
            ["1963-04-14"],
            ["1964-03-29"],
            ["1965-04-18"],
            ["1966-04-10"],
            ["1967-03-26"],
            ["1968-04-14"],
            ["1969-04-06"],
            ["1970-03-29"],
            ["1971-04-11"],
            ["1972-04-02"],
            ["1973-04-22"],
            ["1974-04-14"],
            ["1975-03-30"],
            ["1976-04-18"],
            ["1977-04-10"],
            ["1978-03-26"],
            ["1979-04-15"],
            ["1980-04-06"],
            ["1981-04-19"],
            ["1982-04-11"],
            ["1983-04-03"],
            ["1984-04-22"],
            ["1985-04-07"],
            ["1986-03-30"],
            ["1987-04-19"],
            ["1988-04-03"],
            ["1989-03-26"],
            ["1990-04-15"],
            ["1991-03-31"],
            ["1992-04-19"],
            ["1993-04-11"],
            ["1994-04-03"],
            ["1995-04-16"],
            ["1996-04-07"],
            ["1997-03-30"],
            ["1998-04-12"],
            ["1999-04-04"],
            ["2000-04-23"],
            ["2001-04-15"],
            ["2002-03-31"],
            ["2003-04-20"],
            ["2004-04-11"],
            ["2005-03-27"],
            ["2006-04-16"],
            ["2007-04-08"],
            ["2008-03-23"],
            ["2009-04-12"],
            ["2010-04-04"],
            ["2011-04-24"],
            ["2012-04-08"],
            ["2013-03-31"],
            ["2014-04-20"],
            ["2015-04-05"],
            ["2016-03-27"],
            ["2017-04-16"],
            ["2018-04-01"],
            ["2019-04-21"],
            ["2020-04-12"],
            ["2021-04-04"],
            ["2022-04-17"],
            ["2023-04-09"],
            ["2024-03-31"],
        ];
    }
}