How can I define checksum for attachments which can be downloaded from my website?

<?php
   // File verification tests by Angelo Mandato (angelo [at] mandato {period} com)
 
   // __crc32_file() is very slow, you can uncomment to test for yourself.
   //require_once('crc32_file.php');
   // Copy and paste the contents of the crc32_file() code found on
   // the php.net crc32 PHP manual page in a new file and save
   // as crc32_file.php in the same directory as this script.
 
   // Get microseconds
   function GetMicrotime()
   {
       list($usec, $sec) = explode(" ", microtime());
       return ((float)$usec + (float)$sec);
   }
 
   // file_crc() - function to test
   function file_crc($file)
   {
       $file_string = file_get_contents($file);
       $crc = crc32($file_string);
       return sprintf("%u", $crc);
   } 
   $Methods = array('sha1_file()', 'md5_file()', 'file_crc()');
   if( function_exists('__crc32_file') )
       $Methods[] = '__crc32_file()';
   $directory = '/path/to/directory/'; // Don't forget trailing backslash.
   $files = scandir($directory);
 
   for( $method_index = 0; $method_index < count($Methods); $method_index++ )
   {
       $start_time = GetMicrotime();
       while( list($index,$file) = each($files) )
       {
           if( $file != '.' && $file != '..' && is_file($directory.$file) )
           {
               switch( $method_index )
               {
                   case 0: { // sha1_file()
                       $value = sha1_file($directory.$file);
                   }; break;
                   case 1: { // md5_file()
                       $value = md5_file($directory.$file);
                   }; break;
                   case 2: { // file_crc()
                       $value = file_crc($directory.$file);
                   }; break;
                   case 3: { // __crc32_file()
                       $value = __crc32_file($directory.$file);
                   }; break;
               }
           }
           else // It is not part of our test results, lets remove it from the array
           {
               unset($files[$index]);
           }
       }
       $end_time = GetMicrotime();
       echo sprintf("%s took %.03f seconds to calculate %d files.\n", $Methods[$method_index], $end_time-$start_time, count($files) );
       reset($files); // Reset pointer in array
   }
   echo "file verification tests completed.\n";
?>