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 using PHP from the command line.
Note: This extension is not available on Windows platforms.
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.
You can also use this functions with the libedit library, a non-GPL replacement for the readline library. The libedit library is BSD licensend and available for download from http://sourceforge.net/projects/libedit/.
To use this functions you must compile the CGI or CLI version of PHP with readline support. You need to configure PHP --with-readline[=DIR]. In order you want to use the libedit readline replacement, configure PHP --with-libedit[=DIR].