diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..bf51689
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/.phpunit.cache/
diff --git a/phpunit.xml b/phpunit.xml
index 9f8ad16..7b86f75 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -1,30 +1,14 @@
 <?xml version="1.0"?>
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
-    backupGlobals="false" backupStaticAttributes="false"
-    beStrictAboutChangesToGlobalState="true" beStrictAboutOutputDuringTests="true" beStrictAboutResourceUsageDuringSmallTests="true" beStrictAboutTodoAnnotatedTests="true" beStrictAboutCoversAnnotation="false"
-    bootstrap="tests/bootstrap.php"
-    cacheResultFile=".phpunit.cache/test-results"
-    cacheResult="false"
-    colors="true"
-    convertErrorsToExceptions="true" convertDeprecationsToExceptions="true" convertNoticesToExceptions="true" convertWarningsToExceptions="true"
-    enforceTimeLimit="true"
-    executionOrder="depends,defects"
-    failOnRisky="true"
-    failOnWarning="true"
-    forceCoversAnnotation="false"
-    processIsolation="true"
-    stopOnError="true" stopOnFailure="true" stopOnIncomplete="true" stopOnSkipped="true" stopOnRisky="true"
-    testdox="true"
-    timeoutForSmallTests="1" timeoutForMediumTests="10" timeoutForLargeTests="60"
-    verbose="true">
-    <testsuites>
-        <testsuite name="tests">
-            <directory>tests/</directory>
-        </testsuite>
-    </testsuites>
-    <coverage>
-        <include>
-            <directory suffix=".php">src</directory>
-        </include>
-    </coverage>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.4/phpunit.xsd" backupGlobals="false" beStrictAboutChangesToGlobalState="true" beStrictAboutOutputDuringTests="true" bootstrap="tests/bootstrap.php" cacheResult="false" colors="true" enforceTimeLimit="true" executionOrder="depends,defects" failOnRisky="true" failOnWarning="true" processIsolation="true" stopOnError="true" stopOnFailure="true" stopOnIncomplete="true" stopOnSkipped="true" stopOnRisky="true" testdox="true" timeoutForSmallTests="1" timeoutForMediumTests="10" timeoutForLargeTests="60" cacheDirectory=".phpunit.cache" backupStaticProperties="false" requireCoverageMetadata="false" beStrictAboutCoverageMetadata="false">
+  <testsuites>
+    <testsuite name="tests">
+      <directory>tests/</directory>
+    </testsuite>
+  </testsuites>
+  <coverage/>
+  <source>
+    <include>
+      <directory suffix=".php">src</directory>
+    </include>
+  </source>
 </phpunit>
diff --git a/src/MD_STD_IN.php b/src/MD_STD_IN.php
index ef9bdc9..0c004e1 100644
--- a/src/MD_STD_IN.php
+++ b/src/MD_STD_IN.php
@@ -392,6 +392,27 @@ final class MD_STD_IN {
 
     }
 
+    /**
+     * Validates a ZIP code.
+     *
+     * @param string $input Input string.
+     *
+     * @return string
+     */
+    public static function validate_zip_code(string $input):string {
+
+        if (($input = trim($input)) === "") {
+            return "";
+        }
+
+        if (\mb_strlen($input) > 7) {
+            throw new MDgenericInvalidInputsException("ZIP code is too long");
+        }
+
+        return $input;
+
+    }
+
     /**
      * Returns an UTF8 version of a string.
      *
diff --git a/tests/MD_STD_IN_Test.php b/tests/MD_STD_IN_Test.php
index 9226d31..5425818 100644
--- a/tests/MD_STD_IN_Test.php
+++ b/tests/MD_STD_IN_Test.php
@@ -318,6 +318,8 @@ final class MD_STD_IN_Test extends TestCase {
         self::expectException(MDgenericInvalidInputsException::class);
         MD_STD_IN::validate_longitude("");
         self::expectException(MDCoordinateOutOfRange::class);
+        MD_STD_IN::validate_longitude("1900");
+        self::expectException(MDCoordinateOutOfRange::class);
         MD_STD_IN::validate_longitude(1900);
         self::expectException(MDCoordinateOutOfRange::class);
         MD_STD_IN::validate_longitude(-1900);
@@ -346,6 +348,8 @@ final class MD_STD_IN_Test extends TestCase {
         self::expectException(MDgenericInvalidInputsException::class);
         MD_STD_IN::validate_latitude("");
         self::expectException(MDCoordinateOutOfRange::class);
+        MD_STD_IN::validate_latitude("1900");
+        self::expectException(MDCoordinateOutOfRange::class);
         MD_STD_IN::validate_latitude(1900);
         self::expectException(MDCoordinateOutOfRange::class);
         MD_STD_IN::validate_latitude(-1900);
@@ -378,6 +382,24 @@ final class MD_STD_IN_Test extends TestCase {
 
     }
 
+    /**
+     * Function for testing validate_zip_code().
+     *
+     * @small
+     * @covers \MD_STD_IN::validate_zip_code
+     *
+     * @return void
+     */
+    public function test_validate_zip_code():void {
+
+        self::assertEquals("", MD_STD_IN::validate_zip_code(""));
+        self::assertEquals("1234", MD_STD_IN::validate_zip_code(" 1234"));
+
+        self::expectException(MDgenericInvalidInputsException::class);
+        MD_STD_IN::validate_zip_code("X094339604");
+
+    }
+
     /**
      * Function for testing ensureStringIsUtf8().
      *