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.

.

so. if you are right look at results of this code

function isInt($value) { return preg_match("/^-?[0-9]+$/",$value); }
for ($y = 0; $y < 100; $y++)
{
  $x = (1/20) - (27/20) * $y;
  if (isInt($x)) 
  {
    echo "x = $x; intval(x) = ".intval($x)."; var_dump(x == 
intval(x)) = ";
    var_dump($x == intval($x));
    echo "\n";
  }
}

first result
// x = -4; intval(x) = -4; var_dump(x == intval(x)) = bool(false)

why?