A context is a set of parameters and
wrapper specific options which modify or enhance the
behavior of a stream. Contexts are created using
stream_context_create() and can be passed to most
filesystem related stream creation functions (i.e. fopen(),
file(), file_get_contents(), etc...).
Options can be specified when calling
stream_context_create(), or later using
stream_context_set_option().
A list of wrapper specific options can be found in the
上下文（Context）選項和參數 chapter.
Parameters can be specified for
contexts using the
stream_context_set_params() function.