Implement DateTimeImmutable::createFromMutable

Summary:Closes #6805
Closes #6846

Reviewed By: JoelMarcey

Differential Revision: D2957341

Pulled By: JoelMarcey

fb-gh-sync-id: caf85837f4d4320d5b2df0839d6a7b34196a850c
shipit-source-id: caf85837f4d4320d5b2df0839d6a7b34196a850c

hphp/system/php/date/datetimeimmutable.php

@@ -99,6 +99,12 @@ public static function createFromFormat(
     return $out;
   }
 
+  public static function createFromMutable(DateTime $datetime) {
+    $out = new DateTimeImmutable();
+    $out->data = clone $datetime;
+    return $out;
+  }
+
   public static function getLastErrors(): array {
     return DateTime::getLastErrors();
   }

hphp/test/slow/ext_datetime/DateTimeImmutable_createFromMutable.php

@@ -0,0 +1,9 @@
+<?php
+$current = "2014-03-02 16:24:08";
+
+$i = DateTimeImmutable::createFromMutable( date_create( $current ) );
+var_dump( $i );
+
+$i = DateTimeImmutable::createFromMutable( date_create_immutable( $current ) );
+var_dump( $i );
+?>

hphp/test/slow/ext_datetime/DateTimeImmutable_createFromMutable.php.expectf

@@ -0,0 +1,13 @@
+object(DateTimeImmutable)#%d (1) {
+  ["data":"DateTimeImmutable":private]=>
+  object(DateTime)#%d (3) {
+    ["date"]=>
+    string(26) "2014-03-02 16:24:08.000000"
+    ["timezone_type"]=>
+    int(3)
+    ["timezone"]=>
+    string(%s) "%s"
+  }
+}
+
+Catchable fatal error: Argument 1 passed to DateTimeImmutable::createFromMutable() must be an instance of DateTime, DateTimeImmutable given in %s/DateTimeImmutable_createFromMutable.php on line 7