Downloading and validating MP3 from URL

[eluser]mddd[/eluser]
I would use the getid3 library from www.getid3.org .
This is a library that will really look into the file itself and find out what encoding it has, how long it is (in seconds), and MUCH more.
Looking at the mime time will not tell you if it is a working mp3, but this library will.

On security: of course you should not execute the file in any way, just analyze it.
And preferably not store it in any place that is directly accessible by the public.