-0 not equals 0 - php

Question

I have a function that calculates a value, which is a float:
function crunch (float $a, float $b):float
{
//do stuff
return $result;
}
function testSomething (float $a, float $b):bool
{
//if $result is -0 that returns false
$result = crunch($a, $b);
return $result === 0;
}
Why is -0 not equal to 0 and how can if safely check if that number is zero, assuming that 0 should be the same as -0?
UPDATE
Since there was the question for more details. I have a class Vec2 which has x() and y() getters, and a method called cross, which looks like that:
public function cross(Vec2 $vec2):float
{
return ($this->_x * $vec2->_y) - ($vec2->_x * $this->_y);
}
Running this code:
$cross = $this->cross($d);
results in that debugger output
and $cross === 0 evaluates to false;

Answer 1

Cast the 0 to float. It's probably failing because 0 as literal is an int, and the result is a float, so === is false because of types.
At least doing something like this, fails like your case (the result is false):
php -r '$a = (float) -0; $b = 0; echo ($a === $b);'
The result is true in this case:
php -r '$a = (float) -0; $b = (float) 0; echo ($a == $b);'

Answer 2

Negative zero isn't a thing. But a negative value smaller than the configured precision for floating point display is.
You can't reliably check direct equivalence between two floats, you can only reasonably check that the difference between two floats is smaller than you care about for a given calculation.
eg:
function float_equiv(float $a, float $b, float $epsilon=NULL) {
// default to PHP's configured display precision
$epsilon = $epsilon ?: pow(10, -1*ini_get('precision'));
if( abs($a - $b) < $epsilon ) {
return true;
}
return false;
}

Answer 3

Just to illustrate the accepted answer by #monstercode
Math logic and computer logic are separated entities.
We intuitively know that -0 = 0 but in computer terms the sign of a number is stored separately from the number (like a meta data) - this is what floats do.
In that case an integer is compared to a float.
$result_1 = 1234*0;// remains an integer
$result_2 = -1234*0;// remains an integer
$result_3 = -1.234*0;// becomes a float
$result_4 = 1.234*0; // becomes a float
var_dump($result_1 === 0); // true
var_dump($result_2 === 0); // true
var_dump($result_3 === 0); // false
var_dump($result_4 === 0); // false