The uri_escape() function takes an optional second
           argument that overrides the set of characters that are
           to be escaped.  The set is specified as a string that
           can be used in a regular expression character class
           (between [ ]).  E.g.:

             "\x00-\x1f\x7f-\xff"          # all control and hi-bit characters
             "a-z"                         # all lower case characters
             "^A-Za-z"                     # everything not a letter