PHP 5.6.15 发布 Bug 修复版本

PHP 5.6.15 发布，此版本是个维护版本，主要是 bug 修复，现已提供下载，Windows 二进制包现已提供在 windows.php.net/download/。

详细改进列表：

Core:
- Fixed bug #70681 (Segfault when binding $this of internal instance method to null).
- Fixed bug #70685 (Segfault for getClosure() internal method rebind with invalid $this).
Date:
- Fixed bug #70619 (DateTimeImmutable segfault).
Mcrypt:
- Fixed bug #70625 (mcrypt_encrypt() won’t return data when no IV was specified under RC4).
Mysqlnd:
- Fixed bug #70384 (mysqli_real_query():Unknown type 245 sent by the server).
- Fixed bug #70572 segfault in mysqlnd_connect.
Opcache:
- Fixed bug #70632 (Third one of segfault in gc_remove_from_buffer).
- Fixed bug #70631 (Another Segfault in gc_remove_from_buffer()).
- Fixed bug #70601 (Segfault in gc_remove_from_buffer()).
- Fixed compatibility with Windows 10 (see also #70652).