Composer不能要求PHP简单的HTML DOM解析器，但函数未定义

I am trying to use the Simple HTML DOM Parser package with composer but I am getting the error:

Fatal error: Uncaught Error: Call to undefined function file_get_html()

when I try to call the file_get_html() function.

I successfully installed the package through composer and I can even see the package in my composer/installed.json file. Running composer update returns:

Nothing to install or update

I have of course called:

require 'vendor/autoload.php';

at the top of my file and all of my other packages work fine. I know I could just require it directly instead of using composer and autoload but was wondering why this was happening.