Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
 
Thank you for your interest in PHP.

aharvey, the problem is not related with strings.

I better should write the code like this:

    // this works bad:
    $sum = 4926.11;
    $sum = round($sum, 2) * 100;
    var_dump($sum);                // float(492611)

    $sum = (integer) $sum;
    var_dump($sum);                // int(492610)

    // the first numbers, the other round() operation, works good
    $sum = 4926.11;
    $sum = round($sum * 100);
    var_dump($sum);                // float(492611)

    $sum = (integer) $sum;
    var_dump($sum);                // int(492611)

You probably haven't noticed the main point. After operations on variable with round() function variables have the same representation, float(492611). But the subsequent type conversion to integer returns different values, int(492610) and int(492611).