Add functions for transferring dates to ints and vice versa

tests/MD_STD_Test.php

@@ -0,0 +1,62 @@
+<?PHP
+/**
+ * Tests for MD_STD.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Tests for MD_STD.
+ */
+final class MD_STD_Test extends TestCase {
+
+    /**
+     * Returns sample dates and their equivalent integer values according
+     * to MD_STD::date_to_int().
+     *
+     * @return array<string, array{0: string, 1: integer}>
+     */
+    public static function date_to_int_provider():array {
+
+        $values = [
+            ["2022-01-01", 20220101],
+            ["0022-01-01", 220101],
+            ["-0022-01-01", -220101],
+            ["-2022-01-01", -20220101],
+            ["-0001-01-01", -10101],
+            ["-0000-01-01", -101],
+            ["0000-01-01", 101],
+        ];
+
+        $output = [];
+        foreach ($values as $value) {
+            $output[$value[0]] = $value;
+        }
+        return $output;
+
+    }
+
+    /**
+     * Checks if dates can be translated to int and back.
+     *
+     * @dataProvider \MD_STD_Test::date_to_int_provider
+     *
+     * @param string  $date        Date to translate.
+     * @param integer $expectedInt Expected integer value for it.
+     *
+     * @return void
+     */
+    public function test_date_to_int(string $date, int $expectedInt):void {
+
+        $toInt = MD_STD::date_to_int($date);
+        $toStr = MD_STD::int_to_date($toInt);
+
+        self::assertEquals($expectedInt, $toInt);
+        self::assertEquals($date, $toStr);
+
+    }
+
+}