Description:
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My suggestion is a new magic method called __operator($op, $rightValue) or (if that might be the better way) __operator($leftValue, $op)

If defined, a class should be able to handle an operation, like "$classA + $classOrValueB". What the method returns is the result of that of that operation.

Reproduce code:
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// first way
class A1
{
    // the method of the instance of $a1
    public function __operator($operator, $rightValue)
    {
        var_dump($rightValue);    // 'Hi, All!'
        var_dump($operator);      // '.'

        return '666 . ' . $rightValue;
    }
}

// second way
class A2
{
    // the method of the instance of $a2
    public function __operator($leftValue, $operator)
    {
        var_dump($leftValue);    // 'Hello, World!'
        var_dump($operator);     // '+'

        return '777 + ' . $leftValue;
    }
}

// 1st way
$a1 = new A1();
$c1 = $a1 . 'Hi, All!';         // '666 . Hi, All!'
                                // the result of __operator() method

// 2nd way
$a2 = new A2();
$c2 = 'Hello, World!' + $a2;    // '777 + Hello, World!'
                                // the result of __operator() method


Expected result:
----------------
$c1 = '666 . Hi, All!';
$c2 = '777 + Hello, World!';

Actual result:
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