onchange event of select [duplicate]

I have a PHP program that in the first select returns the directories of my root (I did it in PHP), but in the onchange event of this select I want to populate the other select with the subdirectories of the first one and for that I only researched to do this with javascript . What function should I use to return these sub directories? Each time I select a directory from the first it returns the subs in the second.

                                           Select a directory:                                      $ folders ";                         $ i ++;                     }                     ? >                                                            Select a month:                                      $ folders ";                         $ i ++;                     }                     ? >                                                OK