republic acts and presidential decrees are legislative enactment the difference however is that Republic Acts were made by both the Senate and House of Representatives while Presidential Decrees are of Marcos dictatorial regime through the declaration of Martial Law, he made laws in his exercise of the supposed emergency power (1971-1986). If you want to look for a specific topic you should have at least the year of its approval. They are arrange based on the year, better you should know the number of the law you are looking for or the topic it discuss, i.e. RA 8292 Higher Education Act. You can check www.lawphil.net or google chan robles virtual library,under the topic STATUTES for the complete list of Republic Acts, Commonwealth Act, Presidential Decrees, for a background, the Philippines was a former colony of the US and the laws made are classified according to the period it was passed by the legislature, so aside from Republic Acts (RA, 1946,1986-present), Presidential Decrees (PD, 1971-1986), Batas Pambansa (BP, 1978-1986) both PD and BP are legislative enactments, even though that there was an interim parliament or Batasang Pambansa (literally means National Assembly), Marcos retained his legislative power under the 1973 constitution.
You may also come across with a Commonwealth Act or CA (1935-1941), Philippine Commission Act or ACTS (1901-1916). Municipal and provincial "laws" are covered by the Local Government Code or RA 7160. It seems to me that you are from other country?. Provinces and municipalities enact ordinances, they are not published in the official gazette unlike laws passed by the congress. the equivalent of the United States Federal law is the Republic Act, however we don't have State laws, ordinance pass by the provincial and municipal board are not, in strict legal sense "law", because under the 1987 constitution only the Congress of the Philippines has the power of legislation. Hope this help you to understand a bit of the sources of our law and its periodization.