One letter or not?

Is this one letter or a series of letters put together?  Could this be parts of a series of letters all written to the Philippians?  We know that Paul wrote several letters to the Corinthians and the Thessalonians.  Thus, this may be a compilation of fragments from three separate letters from Paul to the church community in Philippi.  Thus, Philippians may consist of three separate letter fragments.  The first letter would be the first two chapters.  The second letter would be chapter three, and the final part or the third letter would be chapter four.  Each section has unique themes.  In support of the idea that Philippians is a composite work, there are the abrupt shifts in tone and topic within the text. These letter fragments may have been edited into a single document by the first collector of the Pauline corpus, although there is no clear consensus among scholars regarding who this initial collector may have been, or when the first collection of Pauline epistles may have been made.  Nevertheless, many scholars continue to argue for the unity of Philippians.  There is a general consensus that Philippians consists of authentically Pauline material, and that the epistle is a composite of multiple letter fragments from Paul to the church in Philippi.