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.

# php -n -dprecision=32 -r '$a=-0.1; var_dump($a);var_dump(round($a));'
float(-0.10000000000000000555111512312578)
float(-0)

Yes I was aware of the limited precision.
But I'm still wondering if it is a good idea to have a positive and a negative zero as output of rounding operations that both should be mathematically the same zero which has noting to do with a limited float precision. Rounding is not like taking a left or right limit to the next integer. It also inherits problems in some cases e.g.: 

echo (((round(0.1).'EUR') == (round(-0.1).'EUR')) ? 'EQUAL' : 'NOT EQUAL');

gives: "NOT EQUAL"

MATLAB for example also doesn't have a negative zero here.

>> digits(32)
>> sym(-0.1,'d')
 
ans =
 
-.10000000000000000555111512312578
 
>> sym(round(-0.1),'d')
 
ans =
 
0
 
>>