Okay let us examine this logically and in regards to the powers at play.
The Republic of the Philippines is a strategic ally of the United States and is a participating member of the United States' Pacific Command's CARAT exercise annually. The United States Navy PACOM (Pacific Command: which is based throughout Guam, Japan, Korea, Taiwan, and Hawaii) works closely with the Philippine Combined Fleet in anti-piracy activities as well as worked together to augmenting the fire parity of the Philippine Surface Fleet.
Part of the assurance the Alliance the Philippines has with The United States is a guaranteed Nuclear Umbrella. The United States' policy in this is to defend the territorial sovereignty of the United States and its overseas strategic allies/partners. The Philippines, being a strategic partner since the independence of the Philippines from the United States in July 4, 1946.
Let us accept the honest truth. The United States has vested interest in the Philippines, either we choose to accept this reality, it is still fact. The United States' interest in the Philippines was well illustrated when it invaded the Philippines in 1944 to reclaim the Islands from Japanese occupation, thought the United States had an option to bypass the Philippines, they, however, showed and solidified American-Filipino strategic interests by launching the largest naval invasion in the continuum of Human History.
The American Naval Flotilla that was launched to retake the Philippines included an expeditionary scout force of Marines that numbered over 50,000. The United States entire 6th Army landed in Leyte and poured out and overwhelmed any and all Japanese defenders (all 300,000+). The United States unleashed the United States 5th Air Force that took over the air and paved the way for the liberation of the Philippines.
Japan's entire defense force was wiped out; suffering a casualty rate of 300,000+.
The United States Marines and Army lost 62,000 casualties in retaking the Philippines.
America, willingly, sacrificed over 60,000+ of its boys to free the Philippines.
That is equivalent to 6 Divisions being eradicated for the liberation of the Philippines.
Note that they had the option in bypassing the Philippines as what MacArthur was advised by the High Command, to use the "Island Hopping" tactic, but MacArthur and his staff determined that the Philippines was too important, strategically, to the United States to remain under foreign/enemy control.
By keeping his word to 'Return', he set precedence between the two nations.
Now on the topic of China using nuclear attacks on the Philippines, this is impossible and highly improbably. So long as the Chinese want to prosper and attain economic success, which I am confident that they do, they will evade such a suicidal option.
The United States abides by M.A.D (Mutually Assured Destruction). Meaning if any hostile power launches a nuclear warhead on the United States, its territories abroad, and any of its strategic allies, the United States will uphold its national interest and interests of said ally. This is called a 'Nuclear Umbrella'.
So, if China does happen to launch a tactical Nuclear ICBM (intercontinental ballistic missile) on the Philippines, the United States' response would be to Launch a volley of its own ICBM Thermonuclear Missiles on China. It will vaporize and literally 'Glass' all of China as a result.
America has bases in Basilan and throughout Mindanao. And those bases are recognized as American soil/'territory'. An attack on the Philippines will be translated by the United States Military as an attack on the United States of America, itself.
China isn't stupid enough to do so.
It is one of the only reasons why China has not engaged the Philippine Surface Fleet regarding the Kalayaan Group. China knows that Uncle Sam's Nuclear Submarine Wolf Packs and half of the United States' 18 Super Air Craft Carriers and support ships patrol the pacific. And definitely inside Philippine Waters.

No nation will invade the Philippines. Why would they?
The Philippines has a really, really, really powerful friend.
The American HyperPower.
lol
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