$valid_extensions = array('jpg', 'gif', 'png');
 
$file_name  = basename($_FILES['userfile']['name']);
$_file_name = explode('.', $file_name);
$ext        = $_file_name[ count($_file_name) - 1 ];
 
if( !in_array($ext, $valid_extensions) ) {
 /* This file is invalid */
}