Increment on "__toString"

Question

I am not sure what the title should be, but the code should explain it better:

class Group {
    private $number = 20;

    public function __toString() {
        return "$this->number";
    }
}

$number = new Group();
echo $number, PHP_EOL;
echo ++ $number, PHP_EOL;

echo PHP_EOL;

$number = "20";
echo $number, PHP_EOL;
echo ++ $number, PHP_EOL;

echo PHP_EOL;

$number = 20;
echo $number, PHP_EOL;
echo ++ $number, PHP_EOL;

Output:

20
20              <--- Expected 21

20
21

20
21

Any idea why I got 20 instead of 21? Even then the code below works:

$i = null ;
echo ++$i ; // output 1

I know Group is an object that implements __toString , i expected ++ to work with the string from __toString or at least throw an error

Answer 1

The order in which the operations happen is important:

1. The variable will be fetched as an object, it won't be casted to an integer (or something else).

2. This ++ operator increments the lval (the long value) of the zval, but does normally nothing else. The object pointer remains the same. The internal (fast_)increment_function will be called with the zval which has a pointer to the object, which checks for the type first. If it's an object, it does nothing. So when your zval is an object, it is as useful as a no-operation. This won't output any warning.

3. Only then the echo instruction performs a string cast on his arguments: The __toString method is called and returns 20.

4. 20 will be output.