The readline() functions implement an interface to the GNU Readline library. These are functions that provide editable command lines. An example being the way Bash allows you to use the arrow keys to insert characters or scroll through command history. Because of the interactive nature of this library, it will be of little use for writing Web applications, but may be useful when writing scripts meant to be run from a shell.
To use the readline functions, you need to install libreadline. You can find libreadline on the home page of the GNU Readline project, at http://cnswww.cns.cwru.edu/~chet/readline/rltop.html. It's maintained by Chet Ramey, who's also the author of Bash.
To use this functions you must compile PHP with readline support. You need to configure PHP --with-readline.