PHP：有没有办法在对象构造函数中使用全局变量？

I have a php script that is intended to be user-editable, with some options set right at the beginning, including allowable extensions for image files.

Later on, I want to use this in an object constructor to set whether a file is an image or not.

$extensions = ["jpg", "jpeg", "gif", "png"];

class directory_entry {

  function __construct($name) {

    $this->name = $name;

    $extension = pathinfo($name, PATHINFO_EXTENSION);
    $this->is_image = in_array(strtoupper($extension), $extensions); // throws an error

    $this->image = $this->is_image($name);

  }
}

This doesn't work because I can't call $extensions from inside an class(?). Is there a way to do it without including $extensions in every object of this class? Seems nuts to declare objects with

$files[] = new directory_entry($value, $extensions);

and have every instance of this class have a copy of the same array?

The solution that seems to work best for what I'm trying to do (bear in mind that I'm a PHP and OOP n00b) is the globals keyword:

$extensions = ["jpg", "jpeg", "gif", "png"];

class directory_entry {

  function __construct($name) {

    $this->name = $name;

    $extension = pathinfo($name, PATHINFO_EXTENSION);
    global $extensions; //use the global version of $extensions
    $this->is_image = in_array(strtoupper($extension), $extensions); // works!

    $this->image = $this->is_image($name);

  }
}

Why not create a classe for this?

class ExtensionsEnum
{
    const IMAGES = ["jpg", "jpeg", "gif", "png"];
}

Then

use ExtensionsEnum;

class directory_entry {
...
    $this->is_image = in_array(strtoupper($extension), ExtensionsEnum::IMAGES); // throws an error
...
}