Calculate the full birthdate given a CPR number.

_php_function_to_find_birthdate_from_cpr_number.md

cprToBirthdate(string $cpr): ?string

functions.php

<?php

declare(strict_types=1);

/**
 * Calculate the full birthdate given a CPR number.
 * The logic is described in this document.
 *
 * @see https://cpr.dk/media/12066/personnummeret-i-cpr.pdf
 * @param string $cpr
 * @return string|null format:Y-m-d
 */
function cprToBirthdate(string $cpr): ?string
{
    if (!is_numeric($cpr) || !$cpr || strlen($cpr) !== 10) {
        return null;
    }

    $day = $cpr[0] . $cpr[1];
    $month = $cpr[2] . $cpr[3];
    $year = $cpr[4] . $cpr[5];
    $seventh = (int) $cpr[6];

    switch ($seventh) {
        case 0;
        case 1;
        case 2;
        case 3;
            $century = '19';
            break;
        case 4;
        case 9:
            $century = (int) $year <= 36
                ? '20'
                : '19';
            break;
        case 5;
        case 6;
        case 7;
        case 8;
            $century = (int) $year <= 57
                ? '20'
                : '18';
            break;
        default:
            return null;
    }

    $fullYear = $century . $year;

    if (!checkdate((int) $month, (int) $day, (int) $fullYear)) {
        return null;
    }

    return "{$fullYear}-{$month}-{$day}";
}

tests.php

<?php

declare(strict_types=1);

require_once '../functions.php';

use PHPUnit\Framework\TestCase;

final class CprToBirthdateTest extends TestCase
{
    /**
     * @covers ::cprToBirthdate
     * @dataProvider casesProvider
     */
    public function testCases(string $cpr, ?string $birthdate): void
    {
        self::assertSame($birthdate, cprToBirthdate($cpr));
    }

    public function casesProvider(): array
    {
        return [
            // Test edges of all the defined combinations of
            // the 5th, 6th and 7th digit.
            //      0
            ['2412000000', '1900-12-24'],
            ['2412990000', '1999-12-24'],

            //      1
            ['2412001000', '1900-12-24'],
            ['2412991000', '1999-12-24'],

            //      2
            ['2412002000', '1900-12-24'],
            ['2412992000', '1999-12-24'],

            //      3
            ['2412003000', '1900-12-24'],
            ['2412993000', '1999-12-24'],

            //      4
            ['2412004000', '2000-12-24'],
            ['2412364000', '2036-12-24'],
            ['2412374000', '1937-12-24'],
            ['2412994000', '1999-12-24'],

            //      5
            ['2412005000', '2000-12-24'],
            ['2412575000', '2057-12-24'],
            ['2412585000', '1858-12-24'],
            ['2412995000', '1899-12-24'],

            //      6
            ['2412006000', '2000-12-24'],
            ['2412576000', '2057-12-24'],
            ['2412586000', '1858-12-24'],
            ['2412996000', '1899-12-24'],

            //      7
            ['2412007000', '2000-12-24'],
            ['2412577000', '2057-12-24'],
            ['2412587000', '1858-12-24'],
            ['2412997000', '1899-12-24'],

            //      8
            ['2412008000', '2000-12-24'],
            ['2412578000', '2057-12-24'],
            ['2412588000', '1858-12-24'],
            ['2412998000', '1899-12-24'],

            //      9
            ['2412009000', '2000-12-24'],
            ['2412369000', '2036-12-24'],
            ['2412379000', '1937-12-24'],
            ['2412999000', '1999-12-24'],

            // Make sure all parts are padded with zero
            ['0101010000', '1901-01-01'],

            // Invalid cases
            ['', null],
            ['0000000000', null],
            'Invalid length' => ['010101000', null],
            'Must be only digits' => ['0101a10000', null],
            'Invalid day' => ['3201010000', null],
            'Invalid month' => ['0113010000', null],
        ];
    }
}