There are two sides of a coin. Other than that, they also have an edge.
Coin edges were originally reeded (or milled) as a security measure to prevent coin clipping. Some people used to shave off the edges of gold and silver coins and later melt the shavings into bullions. This would usually go unnoticed. However, with the grooves at the sides of the coin, coin clipping would become more noticeable.
Current Philippine coins are not intrinsically valuable and perhaps nobody would bother clipping them. There is still, however, another use for the reeded edges.
The edges of Philippine coins are designed to help differentiate the different denominations. Fumbling for the right coin can be challenging in the dark, and more especially for the blind. Sometimes, the five peso coin can be mistaken for the ten peso coin. The size of the 5 and 10 peso coins may be very similar but their edges are very different.
The edge of the 10 peso coin is reeded while the five peso coin's edge is plain. As you go down the denominations, you will see the pattern. One peso is reeded, 25 sentimo is plain, ten cents reeded, and 5 sentimo is plain (The one sentimo is also plain but the 5 sentimo coin has a glaring hole anyway). Running one's fingertips to feel the edges shoudl do the trick.
The next time you're in a dark jeepney and not sure of the coin you're about to hand out as fare, let the edges reassure you.