6-year resident of the Philippines here, I can add a bit more. Upon entering the country at any port, you'll get a 29-day free visa on arrival (generally speaking, though there are exceptions depending on where you're from). The first time you renew at any Bureau of Immigration office, you're eligible for a 30-day extension (they won't let you do it any longer than that). Costs usually run around 3,000php (~$60USD), which includes an "express fee" that may or may not be optional depending upon which office you go to and which teller/cashier you deal with ... it's worth paying though, as it means you'll get your passport back the same day. Arrive early (8am, even though many of the Bureau of Immigration offices--like all government
offices in the PH--are in malls which don't open until 10am, so find a security
guard who can direct you to an entrance that's designated for individuals looking to do business at the government
offices) and expect to wait ~1 to 2 hours depending on how early you arrive.
After your first 30-day extension, you'll be eligible for a longer extension (30 days, 60 days, 90 days), which costs more but not not proportionally more. The first 90-day extension will be more expensive, as they'll require you to register for what they call an ACR card (Alien Certification
of Registration), which requires some biometric nonsense. The fingerprinting (all digital, at least at the Cebu Bureau of Immigration Office) and card issuance isn't too time consuming, but again, does add a few thousand pesos to the cost.
Beyond that, you're free to reapply for 90-day extensions in to perpetuity. If you've applied for more than a few (I believe after 6 months in the PH, if I'm not mistaken), you'll also be required to apply for an Exit Clearance (again, the Bureau of Immigration) at least three days prior to your scheduled date of departure. Failure to do so can result in some hefty fines/bribes needing to be paid when you try to leave the country.
Fines/fees if you miss your cutoff date aren't terrible, as long as you get to it in a reasonable period of time (say, 1 to 2 weeks). There's simple a fine plus a fee associated with an official "letter of exemption" that needs to go to the regional Bureau chief for notarization, though you can't avail of this more than once or twice before you're going to get in trouble.
Mabuhay! And hope all this COVID nonsense clears up so you can make landfall on our fair shores come 2021 - cheers!