Metro Manila · Cash
Cash is the single most important way of paying your bills and carrying money around. Centavos come in 1, 5, 10, 25, 50 coins and Pesos come in 1, 5, 10 coins and 20, 50, 100, 200, 500 and 1000 bills. It is not that uncommon to pay even large sums in cash. Due to the comparatively low value of the highest bill (1000 Pesos are roughly 20 Euros or 22.50 US Dollar), you might easily end up carrying around a large bundle of bills.
You should always keep as many small bills as possible. Especially for cab rides or corner stores (Sari Sari) you should be holding on to your 50 and 100 Peso bills. It is also good to have a couple of coins and 20 Peso bills in your pocket for tricycle rides, street-food, and small tipps (e.g. for people who stop a cab for you).
Centavos are of such low value that most prices outside supermarkets are only in pesos. It is quite common in many supermarkets that cashiers sum up or down the total sum to whole peso amounts.
If you plan to travel outside the Megacity, make sure you have enough cash with you. Depending on where you are going, there might be not ATMs for you to withdraw money. In those places the likelihood of being able to pay with a credit card is also not very high. It is therefore prudent to check out how much money you roughly need, put some of it in your wallet and hide the rest somewhere else, in bags etc.