i used pathinfo() instead and changed the code to this:

function check_extension($filename, $allowed, $case_sensitive = false) {
$file_ext = pathinfo($filename);
$ext = $file_ext['extension'];
if( !$case_sensitive ) {
foreach( array_keys($allowed) as $key ) {
$allowed[$key] = strtolower($allowed[$key]);
}
$file_ext = strtolower($file_ext);
}
if( in_array( $ext, $allowed ) ) return true; else return false;
}

now it works even if the filename contains more then one dot.

1. instead of looping through the array to see if a given extension is in it, just is in_array() : http://us2.php.net/in_array

2. user regular expressions to find the last set of letters after the last period in the file name:
$str = “radical.jpg”;
$matches = null;
preg_match(‘/\w+$/’,strtolower($str),$matches);
var_dump($matches[0]);

I allways do somethink like this:
substr($filename, -3)

Anyway.. good function, i will use it