I didn't know about this rule when I brought in about p17,000 as emergency money, my first trip overseas, and I hadn't confirmed that my new ATM cards worked out of the country. I ticked the form saying that I had more than 10,000, wasn't sure what to do, but decided that honesty was the best policy. Looked around for a money changer before immigration (as is the case in Singapore) but there wasn't any.
Upon getting to customs, I was very apologetic, but the lady was quite nice about it, there wasn't any issues. I wouldn't recommend anyone do this, just use another currency (less than $US10,000) and take the conversion hit.
You were very lucky that you did not have to surrender the excess amount. I heard many stories of people having to pay fine (meaning bribe) for bringing in more than P10,000. Interestingly the fines were usually the amount above the P10,000 so in your case you would have been hit with P7,000 fine.
I know it is stupid (nothing new since this is PI afterall) but you can bring in $10,000 USD but in pesos you can only bring in $250 (P10,000).