OS X ships with PHP5 installed but it does not have
readline() support compiled in. Anybody using PHP from the command-line will want this, as it allows handy things such as tab completion and scrolling through command history using the up arrow.
Thankfully, MacPorts has a readline variant that can be easily installed:
sudo port install php5 +readline
If you execute the command above, apache2 will come along for the ride because it's a default variant for the PHP5 port. If you don't want apache2 (OS X ships with apache2 anyways), modify the command to look like this:
sudo port install php5 -apache2 +readline
Not sure if your PHP install has readline support? Execute this one-liner to find out:
<?php echo function_exists('readline') ? "yes\n" : "no\n"; ?>