<?php
class Foo {
    public $string = '';
    //public $length = 0;

    public function __get (string $name) {
        if($name === 'length') return strlen($this->string);
        $trace = debug_backtrace();
        trigger_error(
            'Undefined property via __get(): ' . $name .
            ' in ' . $trace[0]['file'] .
            ' on line ' . $trace[0]['line'],
            E_USER_NOTICE);
        return null;
    }
    
    public function __isset ( string $name ) : bool {
        if($name === 'length') return true;
        return false;
    }

    public function __debugInfo() : array {
        return [
            'length' => $this->length,
            'string' => $this->string
        ];
    }



    public function addCharsAt(int $index, string $chars) {
        if(strlen($chars) === 0 || $index < 0 || $index > 1 + $this->length) return null;
        $str='';
        $len = $this->length;
        for ($i = 0; $i <= $len;) {
            if ($i === $index) {
                $str .= $chars;
                $index = -1;
                //$this->length += strlen($chars);
            } elseif ($i < $len) {
               $str .= $this->string[$i];
               $i++;
            } else break;
        }
        $this->string=$str;
        return null;
    }


}


$foo = new Foo();


$foo->addCharsAt(0, "test");
var_dump($foo);

$foo->addCharsAt(2, "xx");
var_dump($foo);

$foo->addCharsAt(6, "yy");
var_dump($foo);