$ sudo pear channel-discover pear.firephp.org
$ sudo pear install firephp/FirePHPCore

PHP’s PEAR repository does not come with Leopard but it only takes about five minutes, if that, to install.

Download the PEAR install package:

$ curl http://pear.php.net/go-pear > go-pear.php

Run the installer as superuser:

$ sudo php -q go-pear.php

If your behind a proxy/firewall you can enter the required details when the following prompt appears:

HTTP proxy (http://user:password@proxy.myhost.com:port), or Enter for none:

The PEAR installer will suggest a file layout similar to the following, we need to change the installation prefix (see below):

 1. Installation prefix ($prefix) : /Users/usrname

 2. Temporary files directory     : $prefix/temp

 3. Binaries directory            : $prefix/bin

 4. PHP code directory ($php_dir) : $prefix/PEAR

 5. Documentation base directory  : $php_dir/docs

 6. Data base directory           : $php_dir/data

 7. Tests base directory          : $php_dir/tests

Change the installation prefix to ‘/usr/local’:

1-7, 'all' or Enter to continue: 1

Installation prefix ($prefix) [/Users/awmstrs] : /usr/local

Choose whether or not to install optional bundled packages when the following prompt appears (see PEAR‘s site for more info) :

The following PEAR packages are bundled with PHP: PEAR_Frontend_Web-beta, PEAR_Frontend_Gtk2, MDB2.

Would you like to install these as well? [Y/n] :

Allow the install script to alter the php.ini file or make the changes yourself (see http://clickontyler.com/):

Would you like to alter php.ini </private/etc/php.ini>? [Y/n] : Y

That’s it – as the script says:

The 'pear' command is now at your service at /usr/local/bin/pear

Thanks to http://clickontyler.com/ for pointing me in the right direction.