I tried out on Ubuntu Server.

Check this tutorial: http://www.youtube.com/watch?v=xHjk4mZALSY

You have to install youtube-dl :
sudo apt-get install youtube-dl

If you have problem with running check this link: http://askubuntu.com/questions/431508/unable-to-download-videos-using-youtube-dl-sig-error-how-to-correct

sudo youtube-dl -U

sudo easy_install -U youtube-dl

sudo youtube-dl

My test: sudo youtube-dl – f37 --extract-audio --audio-format mp3 -o "/var/www/mp3 folder/%(title)s.%(ext)s" https://www.youtube.com/watch?v=0w3olGsQ6tQ

f37: audo size setting

… You can set url with PHP:

$validLink = validData($_GET['youtubelink']);

$output = shell_exec('youtube-dl --extract-audio --audio-format mp3 -o "/var/www/mp3 folder/file/%(title)s.%(ext)s" '.$validLink );