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
  Context options and parameters chapter.
 
  Parameters can be specified for
  contexts using the
  stream_context_set_params() function.