Please, I believe I have found a serious bug.  I can reproduce it 100%.

Essentially I have a loop (I think this might be significant?) that checks through some rows in a database and if the value of a row (an amount of money) is less than the total ($total2pay) then the ID for that row is inserted into an array to be cancelled, and the total is reduced by the amount of that row.

The loop works fine, unless there are a number of items which add up exactly to the total.  Then, when subtracting the final item (so we get x - x) instead of 0 we get something like 2.7755575615629E-17.

Below is the problem script and underneath that is the output I get. The script has been expanded out to use more variables to make it clearer and prints some debugging stuff. It's not incredibly efficient on SQL requests, but that's irrelevant.
I will gladly give any more information or debug output on request if it's helpful.  NB I do NOT have the Zend Optimiser running for this.

The database structure of table 'debts' is:
id int(10), owed_to varchar(20), owed_by varchar(20), amount decimal (10,2)

// Let there be initially 3 rows in the db which match the query below, // with amount = "0.71", "0.20" and "0.10" respectively
$res = db_query("SELECT * FROM debts WHERE owed_by='$who_is_paying' AND owed_to='$payee' ORDER BY amount DESC");
// Let $total2pay = 1.01
do {
  while ($row = mysql_fetch_array($res)) {
    if ($row[amount] <= $total2pay) { // cancel item
      $cancel[] = $row[id];					
      print "oldtotal:$total2pay,type ".gettype($total2pay)."<br>";
      $new = $row[amount];
      print "amount2delete:$new, type ".gettype($new)."<br>";
      // BELOW LINE IS THE PROBLEM ONE
      $fish= $total2pay - $new;// deduct it from the total owed
      print "newtotal:$fish,type ".gettype($fish)."<br>";
      $total2pay = $fish;
    }
  }
  // delete the marked items from the database
  if ($cancel) {
    reset($cancel);
    while ($item = each($cancel)) {
      db_query("DELETE FROM debts WHERE id='$item[value]'");
    }
  }

  // retrieve the fresh, updated item list from the databas
	mysql_free_result($res);
  $res = db_query("SELECT * FROM debts WHERE owed_by='$who_is_paying' AND owed_to='$payee' ORDER BY amount DESC");
  $rows = db_num_rows($res);
} while (($total2pay > 0) && ($rows > 0) && $cancel);


The output from this is:

oldtotal:1.01,type string
amount2delete:0.71, type string
newtotal:0.3,type double

oldtotal:0.3,type double
amount2delete:0.20, type string
newtotal:0.1,type double

oldtotal:0.1,type double
amount2delete:0.10, type string
newtotal:2.7755575615629E-17,type double

I have played a bit to try to fix it, and found that if I set $new explicitly to type double just before the problem line, the output is exactly the same, i.e. the output is:

oldtotal:1.01,type double
amount2delete:0.71, type double
newtotal:0.3,type double

oldtotal:0.3,type double
amount2delete:0.2, type double
newtotal:0.1,type double

oldtotal:0.1,type double
amount2delete:0.1, type double
newtotal:2.7755575615629E-17,type double

Another thing: if there is only one row in the database and this adds up to the total, it works fine:

oldtotal:1.01,type string
amount2delete:1.01, type double
newtotal:0,type double

I really hope someone can help me get to the bottom of this!