Amendments to the Constitution can be proposed by one of three methods:
All proposed amendments, regardless of the method of proposal, must be ratified by a majority vote in a national referendum.
There have been five constitutional conventions in Philippine history:
None of the proposed amendments to the 1987 Constitution has succeeded, because none have reached the ratification by referendum stage.
The Official Gazette website on the Philippines Government web portal (open access) contains all Constitutions (as enacted and as amended), from the 1899 Malolos Constitution to the current 1987 Constitution.
The LAWPHiL Project website (open access) contains the 1899 Malolos, 1935, 1972 and 1987 Constitutions.
The International Encyclopedia of Laws: Constitutional Law on Kluwer Law Online (UniMelb staff & student access) contains a useful history and detailed commentary on the Philippines Constitution, including references to cases. Scroll to The Philippines under the 'National Monographs' heading.
Oxford Constitutions of the World (UniMelb staff & student access) contains the full text of the current 1987 Constitution accompanied by concise and useful background commentary, and a bibliography of books and articles.