Deletes filename. Similar to the Unix C unlink() function. 成功回傳TRUE失敗回傳FALSE。
備注: As of PHP 5.0.0 unlink() can also be used with some URL wrappers. Refer to 附錄 L for a listing of which wrappers support unlink().
備注: The context parameter was added as of PHP 5.0.0.
See also rmdir() for removing directories.