This is the thing with floating numbers, just as the manual states:

   It is quite usual that simple decimal fractions like 0.1 or 0.7 cannot be converted into their internal binary counterparts without a little loss of precision.

The follwing works as you would expect it therefore:

<?

   $tree = 366.22;

   $one = 1519.36;
   $two = $one - $tree;

   $four = $two + $tree;
   $difer = $one - $four;
   echo "one " . $one . " equals four " . $four . "<br>";
   if ($four == $one) {
      echo "THEY ARE EQUAL <br>";
   }
   else {
      echo "THEY ARE NOT  EQUAL DIFFERENCE " . $difer . "<br>";
   }
?>

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.