streamwrapper.php

<?php
class MyStreamWrapper {
    private $read = false;

    public function stream_open($path , $mode , $options , &$opened_path) {
        return true;
    }

    public function stream_stat() {
        return [];
    }

    public function stream_read($count) {
        if (!$this->read) {
            $this->read = true;
            return 'Hello world!';
        }

        return false;
    }

    public function stream_eof() {
        return $this->read;
    }
}

stream_wrapper_register('my', MyStreamWrapper::class);
var_dump(file_get_contents('my://hello'));