Question

I’m wondering why is output of the following code:

$a = log(5, 5);
$b = round(log(5, 5));
echo 'a: ';
var_dump($a);
echo '<br>';
echo 'b: ';
var_dump($b);
echo '<br>';
echo ($a == $b) ? 'equal' : 'not equal';

this

a: float(1) 
b: float(1) 
not equal

Thanks to anyone who can explain this to me.

Answer 1

Just to quote the chapter Floating point numbers from the PHP manual:

Warning Floating point precision

Floating point numbers have limited precision. Although it depends on
the system, PHP typically uses the IEEE 754 double precision format,
which will give a maximum relative error due to rounding in the order
of 1.11e-16. Non elementary arithmetic operations may give larger
errors, and, of course, error propagation must be considered when
several operations are compounded.

Additionally, rational numbers that are exactly representable as
floating point numbers in base 10, like 0.1 or 0.7, do not have an
exact representation as floating point numbers in base 2, which is
used internally, no matter the size of the mantissa. Hence, they
cannot be converted into their internal binary counterparts without a
small loss of precision. This can lead to confusing results: for
example, floor((0.1+0.7)*10) will usually return 7 instead of the
expected 8, since the internal representation will be something like
7.9999999999999991118….

So never trust floating number results to the last digit, and do not
compare floating point numbers directly for equality. If higher
precision is necessary, the arbitrary precision math functions and gmp
functions are available.