Don't expose member data in public

example-good-interface-3.php

<?php 
// Example of exposing member data in public
// Coding Horror
class Class
{
  public $student;
  
  public $teacher;
}

// Example of good encapsulation (don't expose member data in public)
class Class
{
  protected $student;
  
  protected $teacher;
  
  public function setStudent($student) {
    $this->student = $student;
  }
  
  public function setTeacher($teacher) {
    $this->teacher = $teacher; 
  }
  
  public function getStudent() {
    return $this->student;
  }
  
  public function getTeacher() {
    return $this->teacher; 
  }
}