It was 2 PHP : 1 USD because it was a fixed exchange rate. The Philippine Coinage Act of 1903 stipulated that it should be worth half a dollar. It was quite overvalued post WW2 and heavy import controls had to be put in place. This lead to a bout of successful Import Substitution Industrialization under Ramon Magsaysay. That didn't last however so they finally floated the peso under Diosdado Macapagal when it went to about 4 PHP : 1 USD.

In 1955, the Philippines had a gross national product (GNP) of $4.7 billion (U.S. dollars), whereas Korea’s GNP was only $2.3 billion; moreover, on a per capita basis the Philippines was roughly twice as rich as Korea. - David C. Kang

It is however true that the Philippines was 2nd only to Japan in development.

By comparison with most underdeveloped countries, the basic economic position of the Philippines is favorable. . . . Through a comparatively high level of expenditure on education, transport, communications, and industrial plant over the past fifty years, the Philippines has achieved a position in the Far East second only to Japan . . . the prospects of the Philippine economy for sustained long-term growth are good. – World Bank, 1957

It really only went haywire when Ferdinand Marcos took power. Although corruption also took place in Asian Tiger's such as Korea, there was no "mutual hostage" situation were politicians were restrained by the need to secure bribes from powerful conglomerates.

A mutual hostage situation often meant that favoured conglomerates could borrow credit at a rate lesser than inflation, which was the case under Park. These conglomerates would then use that money to invest in industrial capital and expand. Marcos on the other hand operated virtually unchallenged with his corruption spiraling out of control.

TLDR: Marcos happened

Source: Crony Capitalism by David C. Kang

I'm not sure I understand the question. Just because American and Filipino currencies had equal numeric value doesn't mean that they had equal global purchasing power and doesn't really reflect the relative power of the Philippines in Asia. Could you elaborate more on why you feel the Philippines would be considered among the most powerful nations in Asia?

I'd reccommend a recent book by Joe Studwell How Asia Works, which compares paths of development of East Asian countries. Basically, Japan, Korea and Taiwan succeded because of comprehensive land reform and state support for export industries. Land reform was constantly promised by politicians in Philippines, but never delivered, as large landowners had too much political power.