clip an uploaded song?

[eluser]newbie boy[/eluser]
this one works...

Code:

exec('C:\ffmpeg\ffmpeg -t 30 -acodec copy -i C:\music\input_song.mp3 C:\music\output_song.mp3');

but when i'm passing the audio file to a variable like...

Code:

exec('C:\wamp\bin\ffmpeg\bin\ffmpeg -t 30 -acodec copy -i $inputfile $outputfile')

it doesn't work...

appreciate the help guys...