Since at the start of 2007 all books will no longer be using the ISBN10, we needed a way to convert all those ISBN10 numbers to the new ISBN13 (or EAN).

function genchksum13($isbn)
{
   $isbn = trim($isbn);
   $tb = 0;
   for ($i = 0; $i <= 12; $i++)
   {
      $tc = substr($isbn, -1, 1);
      $isbn = substr($isbn, 0, -1);
      $ta = ($tc*3);
      $tci = substr($isbn, -1, 1);
      $isbn = substr($isbn, 0, -1);
      $tb = $tb + $ta + $tci;
   }
   
   $tg = ($tb / 10);
   $tint = intval($tg);
   if ($tint == $tg) { return 0; }
   $ts = substr($tg, -1, 1);
   $tsum = (10 - $ts);
   return $tsum;
}
function isbn10_to_13($isbn)
{
   $isbn = trim($isbn);
   if(strlen($isbn) == 12){ // if number is UPC just add zero
      $isbn13 = '0'.$isbn;}
   else
   {
      $isbn2 = substr("978" . trim($isbn), 0, -1);
      $sum13 = genchksum13($isbn2);
      $isbn13 = "$isbn2$sum13";
   }
   return ($isbn13);
}

// usage
echo isbn10_to_13('1234567890'); // returns ISBN13

There are no checks in the code to see if the number is numeric for example, but this should give you a good place to start.