<?php
class C {
    private $prop;
    public function __construct($val) {
        $this->prop = $val;
    }
    /**
     * @return array
     */
    public function __debugInfo() {
        return [
            'propSquared' => $this->prop ** 2,
        ];
    }
}
var_dump(new C(42));

object(C)#1 (1) { ["propSquared"]=> int(1764) }