search a php string for two substrings

Question

I am programming a php program that will allow users to download audio from my website.
To do this they go to www.mysite.com/downloadit.php?file=myfile.mp3 and the download of myfile.mp3 will begin immediately.

There is a problem though: I don't want people to be able to download system files. I am going to solve this by checking if $_GET['file'] contains the substrings .mp3 or .wav. I am trying to do this with the strpos command, but can't get it working. How would I check for more than one substring (.mp3 and .wav) in a string with strpos? Or maybe I should use a different command? Please let me know!
Here is my code so far:

$haystack=$_GET['file'];

$resulting = strpos($haystack, ".mp3");

//if found $resulting will equal 1
//if not found $resulting will equal 0

//if $resulting is less than one then it must be 0

    if($resulting < 1){
    //security breach!
    die("Unauthorized");
}

//assuming we passed the last test, everything went well, we will then continue

    else{
    //code here
}

Thanks to @DoubleSharp i now have this completed code!!!

//if it is found $resulting will not equal 0
//if it is not found $resulting will equal 0

//echo the result
//echo $resulting;

//add a line break
echo "<br/>";
//if $resulting is less than one then it must be 0
//since it is 0 it must mean that it is a breach!
if (preg_match("~\.(mp3|wav)$~i", $haystack))
{
  echo "hi nice file";
}
else
{
  die("bad file");
}
?>

Answer 1

You can use regular expressions to test for multiple values, specifically preg_match(). If you use the pattern \.(mp3|wav)$~i wrapped in a delimiter (~ in this case) it will match strings that end with a literal dot . followed by either mp3 or wav. The $ in the pattern matches the end of the line, and the i modifier at the end tells it to do case insensitive matching, so file.MP3 and file.mp3 will both match.

if ( preg_match("~\.(mp3|wav)$~i", $haystack) ) {
    // it matches
}