构造一个对象

class Tree {
    public $val;
    public $left;
    public $right;

    public function __construct($element) {
        if (!empty($element)) {
            $this->val = $element;
        }
    }
}

$s1 = new Tree('1');
$s2 = new Tree('2');
$s3 = new Tree('3');
$s4 = new Tree('4');
$s5 = new Tree('5');
$s3->left = $s4;
$s3->right = $s5;
$s4->left = $s1;
$s4->right = $s2;

var_dump($s3);