title: How to combine two audio files with one being the background music in PHP?
body: <p>I want to get a single audio file from two user inputs - one for background music and one for audio file. Any Ideas how to do so? </p>
  
  <p><strong>EDIT</strong>:
  Basically, I what I want is the final file be as long as the audio file and having the background music chosen by the user. I do not want to concatenate the file but something like, overlapping of audio files.</p>