if you are looking for a fast function to hash a file, take a look at
http://www.php.net/manual/en/function.hash-file.php
this is crc32 file checker based on a CRC32 guide
it have performance at ~ 625 KB/s on my 2.2GHz Turion
far slower than hash_file('crc32b','filename.ext')
<?php
function crc32_file ($filename)
{
   $f = @fopen($filename,'rb');
   if (!$f) return false;
   
   static $CRC32Table, $Reflect8Table;
   if (!isset($CRC32Table))
   {
      $Polynomial = 0x04c11db7;
      $topBit = 1 << 31;
       
      for($i = 0; $i < 256; $i++) 
      { 
         $remainder = $i << 24;
         for ($j = 0; $j < 8; $j++)
         {
            if ($remainder & $topBit)
               $remainder = ($remainder << 1) ^ $Polynomial;
            else $remainder = $remainder << 1;
         }
         
         $CRC32Table[$i] = $remainder;
         
         if (isset($Reflect8Table[$i])) continue;
         $str = str_pad(decbin($i), 8, '0', STR_PAD_LEFT);
         $num = bindec(strrev($str));
         $Reflect8Table[$i] = $num;
         $Reflect8Table[$num] = $i;
      }
   }
   
   $remainder = 0xffffffff;
   while ($data = fread($f,1024))
   {
      $len = strlen($data);
      for ($i = 0; $i < $len; $i++)
      {
         $byte = $Reflect8Table[ord($data[$i])];
         $index = (($remainder >> 24) & 0xff) ^ $byte;
         $crc = $CRC32Table[$index];
         $remainder = ($remainder << 8) ^ $crc;
      }
   }
   
   $str = decbin($remainder);
   $str = str_pad($str, 32, '0', STR_PAD_LEFT);
   $remainder = bindec(strrev($str));
   return $remainder ^ 0xffffffff;
}
?>

<?php
$a = microtime(); 
echo dechex(crc32_file('filename.ext'))."\n"; 
$b = microtime(); 
echo array_sum(explode(' ',$b)) - array_sum(explode(' ',$a))."\n";
?>
Output:
ec7369fe
2.384134054184 (or similiar)