p() is the main method, and d() works the same way, except that it calls the die() method instead of returning the variable:
of that, PrestaShop defines the ppp() and ddd() methods, which are respectively the aliases of p() and d(). They work exactly the same, but are often easier to search and find in a huge block of code.
ppp()
ddd()
These debug methods are not activated by default. To activate them, you must enable the Debug mode, by setting _PS_MODE_DEV_ to true (see above).
http://doc.prestashop.com/display/PS16/Setting+Up+Your+Local+Development+Environment