square.php

<?php
class Square {
  private $height;
  private $width;

  public function setLength($length) {
    $this->height = $length;
    $this->width= $length;
  }

  public function getHeight() {
    return $this->height;
  }

  public function getWidth() {
    return $this->width;
  }
}

$square = new Square();
$square->setLength(100);
echo $square->getHeight(); // 100
echo $square->getWidth(); // 100