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.

.

number_format should 'format' numbers, not recalculate, or? thats how i understand the function.

Yes, that is all number_format does.  But, you asked it for 50 digits of 
precision for 1234.560000 which just isn't possible for computers to represent 
given the way they they handle floating point numbers.  It has nothing to do 
with number_format.  You will see the same if you do:

ini_set('precision',50);
echo 1234.560000;

If you need super-precise floating point, you can't use the computers built-in 
floating point support.  You will need to use http://php.net/gmp instead.  The 
big drawback is that it is much much slower than the computer's native floating 
point support.