sum inside if statement in PHP

somebody can explain me why this code:

$a = 0.1;
$b = 0.2;

if ($a + $b == 0.3) {
    echo "OK";
} else {
    echo "KO";
}

returns KO?

I don't understand why the sum result is different the float 0.3, considering that:

var_dump($a + $b);

returns: float(0.3)

The only hypothesis I have is that the comparison is made between only $b and 0.3 but the doubt remains because also in this case:

if ( ($a + $b) == 0.3) {

I get KO..