Support DateTimeImmutable as cell value

src/PhpSpreadsheet/Cell/DefaultValueBinder.php

@@ -2,7 +2,7 @@
 
 namespace PhpOffice\PhpSpreadsheet\Cell;
 
-use DateTime;
+use DateTimeInterface;
 use PhpOffice\PhpSpreadsheet\RichText\RichText;
 use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
 
@@ -16,14 +16,14 @@ class DefaultValueBinder implements IValueBinder
      *
      * @return bool
      */
-    public function bindValue(Cell $cell, $value = null)
+    public function bindValue(Cell $cell, $value)
     {
         // sanitize UTF-8 strings
         if (is_string($value)) {
             $value = StringHelper::sanitizeUTF8($value);
         } elseif (is_object($value)) {
             // Handle any objects that might be injected
-            if ($value instanceof DateTime) {
+            if ($value instanceof DateTimeInterface) {
                 $value = $value->format('Y-m-d H:i:s');
             } elseif (!($value instanceof RichText)) {
                 $value = (string) $value;

src/PhpSpreadsheet/Cell/IValueBinder.php

@@ -12,5 +12,5 @@ interface IValueBinder
      *
      * @return bool
      */
-    public function bindValue(Cell $cell, $value = null);
+    public function bindValue(Cell $cell, $value);
 }

tests/PhpSpreadsheetTests/Cell/DefaultValueBinderTest.php

@@ -3,6 +3,7 @@
 namespace PhpOffice\PhpSpreadsheetTests\Cell;
 
 use DateTime;
+use DateTimeImmutable;
 use PhpOffice\PhpSpreadsheet\Cell\Cell;
 use PhpOffice\PhpSpreadsheet\Cell\DataType;
 use PhpOffice\PhpSpreadsheet\Cell\DefaultValueBinder;
@@ -11,9 +12,9 @@ use PHPUnit\Framework\TestCase;
 
 class DefaultValueBinderTest extends TestCase
 {
-    protected $cellStub;
+    private $cellStub;
 
-    protected function createCellStub()
+    private function createCellStub()
     {
         // Create a stub for the Cell class.
         $this->cellStub = $this->getMockBuilder(Cell::class)
@@ -53,6 +54,7 @@ class DefaultValueBinderTest extends TestCase
             ['-123.456'],
             ['#REF!'],
             [new DateTime()],
+            [new DateTimeImmutable()],
         ];
     }