Assigning a file number

I need to assign a unique file number 50-20131103-0001 Ussers work in secteur 50 Today is 20131101 The first file number possible today is 0001 (of probably no more than 9999. How should I build this? Should I create a varchar in MySQL and stock it all in one field? or have three separate fields? secteur: '50' (INT) default=50 date: (DATESTAMP) filenumber 0001 (INT) Then here comes the PHP question? When a user opens up my php page, my code will automatically assign $filenumber sequentially. So how do I check if $filenumber = 0001 exists already? Using a while or if seems like a lot of work. Swatting a fly with a baseball bat. There must be an easier trick, maybe a MySQL function to says if 0001 is taken, check and see if 0002 is available ++ until one is free. Then INSERT it so my user can enter his data. John