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）选项和参数 chapter.
Parameters can be specified for
contexts using the
stream_context_set_params() function.
Two important terms:
- An *option* is a protocol-specific setting, e.g. "method" (get, post, put...) if you are using HTTP or "callback function to be called when inserting a document" in MongoDB.
- A *parameter* is a settings that's common to all protocols. As of 2015 only one parameter got implemented ("notification").
While these words might look vague they are used coherently throughout the stream feature, documentation included.