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.

I think I understand what you are saying - however this is essentially the same problem - without any float to string processing:

<?php
// Outputs bool(true)
var_dump(3.14159265358979311599796 === 3.14159265358979323846264);
?>

And if this is expected because of the limitations with the precision - should the precision of PHP be limited to 15 decimal places?

Sorry, I was being a bit slack: we get enough floating point questions that my first comment's actually a canned answer. :)

You're exactly right: it's just that floating point values don't have enough precision to go beyond a certain point -- the float page in the manual says it's usually about 14 decimal digits, which lines up almost exactly with what you've just tested.

As for whether PHP should be limited, it's basically a reality of current computing that decimal values are represented as floating point values with limited precision -- fixed point arithmetic would potentially be more accurate, but much, much slower, since you'd no longer be getting any help from the hardware FPU, and for most purposes, 64-bit floating point values are good enough.

Haha, unfortunately I'm sure I won't be the last to ask about floating point values!

Thanks very much for clearing this up.