Assume we have a person class.  The class constructor takes the persons full name and we have a getter and setter for the full name.

<?php
class Person
{
    protected $fullName;
 
    public function __construct($fullName)
    {
        $this->setFullName($fullName);
    }
 
    public function setFullName($fullName)
    {
        $this->_fullName = $fullName;
    }
 
    public function getFullName()
    {
        return $this->_fullName;
    }
}

There is also a method that parses apart the persons full name into separate parts. The naive approach:

    public function parseFullName()
    {
        $nameParts = explode(' ', $this->getFullName());
        ...
    }

This code works fine, but think about how we will write a test for this function.  We have to first set the full name using a setter so the parseFullName method can retrieve that value with a getter.  This violates one of the principle rules of unit testing: testing individual units of code.  If there is a bug in the getter or setter functions, they may inadvertently affect my test.  This is a very real issue when using the magic __get and __set methods. It also means more work to setup your tests because you have to keep in mind an order of operations when testing.

The better approach:

    public function parseFullName($fullName)
    {
        $nameParts = explode(' ', $fullName);
        ...
    }

Notice how the parseFullName function is being told what to parse rather than asking what to parse.  This subtle change now allows us to truly test this individual unit of code with the least amount of outside environment interaction.