Nor can you specify -0 as a float literal:
var_dump(-0.);
float(0)

The zero negative could appear also in the simplest expressions, yielding quite unexpected results, as in

$zn = -1 * 0.0;
echo $zn; ==> -0

Under PHP several int expressions might result in a float value, and most programmers are unaware that their "exact" calculations are made in the quite obscure domain of the approximated floating-point numbers, possibly with safety and security related issues.

Since $zn==0.0 and $zn===0.0 are both true, the only way I have found to detect the zero negative value is by comparing its representation as a string:

if( $f === 0.0 and "$f" === "-0" )
    echo "WARNING: it's zero negative!";

I've found that this break was introduced in 5.2.3, http://3v4l.org/MFa5s 
I'll try and investigate further.

This is a bug, marking as such.

Still exists in PHP 7, annoyingly.