php OOP: Using eval() to build a string to access $GLOBALS - php

Question

The following code works and does what I want, but I'm pretty sure I'm doing something dumb\awful.
I'm learning OOP and there is a tutorial I started to follow that used a "Config" class to setup some parameters for the program to use. I've noticed something similar in other tutorials. This tutorial though only included a method to retrieve the configuration (it used the $GLOBALS array) not to update it during the run time of the program. I attempted to add this functionality, but resorted to using eval() which I think is a nono? Also it was never explained in the tutorial why the $GLOBALS array was used instead of just using a static variable so I'm confused about that as well.
Here is init.php which gets included in files needing to access the config options:
<?php
$GLOBALS['config'] = array(
'mysql' => array(
'host' => '127.0.0.1',
'username' => 'root',
'password' => '123456',
'db' => NULL
),
'shell' => array(
'exe' => 'powershell.exe',
'args' => array(
'-NonInteractive',
'-NoProfile',
'-NoLogo',
'-Command'
)
)
);
spl_autoload_register(function($class){
require_once 'classes/' . $class . '.php';
});
This is the Config.php class which has a get and (my) set method to access the config array. For the set method I build a string like "$GLOBALS['config']['someConfig']['someSubConfig'] = 'newVal';" and use eval to execute it. Ultimately I use it in the program like Config::set('mysql/host','zzzzz');
<?php
class Config {
public static function get($path=NULL) {
//return all configs if not specified
$config = $GLOBALS['config'];
if($path) {
//parse path to return config
$path = explode('/', $path);
foreach($path as $element) {
if(isset($config[$element])) {
$config = $config[$element];
} else {
//if config not exist
$config = false;
}
}
}
return $config;
}
public static function set($path=NULL,$value=NULL) {
if($path) {
//parse path to return config
$path = explode('/', $path);
//Start code string for eval
$globalPosition = '$GLOBALS['."'config'".']';
foreach($path as $element) {
$globalPosition .= "['$element']";
}
$globalPosition .= "='$value';";
//End code string
eval($globalPosition);
var_dump($GLOBALS);
}
}
}

Answer 1

First of all, here are a few caveats:
Global variables are rarely a good idea, especially in OOP design (mainly because they couple code very tightly).
Please don't use eval().
You can quite easily modify your code to set the variable (by reference using =&) without having to use eval() at all. For example:
public static function set($path = null,$value = null)
{
if($path)
{
//parse path to return config
$path = explode('/', $path);
//Start code string for eval
$setting =& $GLOBALS['config'];
foreach($path as $element)
{
$setting =& $setting[$element];
}
$setting = $value;
var_dump($GLOBALS);
}
}