According to the FAQs on the extension of stay at the Bureau of Immigration website:
I am a foreign national under a Temporary Visitor’s Visa, I can stay legally in the Philippines for thirty (30) days however I want to extend my stay, how do I go about this?
Foreign nationals who are admitted with an initial stay of thirty (30) days may apply for a visa waiver first, granting an additional stay of twenty nine (29) in the Philippines. Thereafter, you may apply for one (1) month, two (2) months or six (6) months extensions at least one week prior to the expiration of your valid stay.
What are the requirements for me to extend my Temporary Visitor’s Visa?
For visa extension requirements, kindly bring your passport and submit a filled out visa extension form (TVS-C-VE-2016) at any BI offices.
This also applies to Indian nationals who may have been granted a visa for 14 days as per the Visa Application Guidelines published by the Embassy of the Philippines in New Delhi.
Please see section on Length of stay :
If you wish to stay longer than the maximum allowable period, you can request for an extension through the Visa Extension Section, Philippine Bureau of Immigration, Manila.
A list of offices of the Bureau of Immigration can be found here.
Wish you good luck!