Up 1 parent directory level

<link href="../styles.css" rel="stylesheet" type="text/css" />

<?php require('../header.php');?>

PHP Website Root directory

  if (is_file($_SERVER['DOCUMENT_ROOT'] . '/myfile.jpg'))
    echo "FILE EXISTS";

This will give you the root directory of the current website (not the server root), e.g. ‘/public_html/’.

If you want to access a folder above it you can do this:

  require_once $_SERVER['DOCUMENT_ROOT'] . '/../vendor/autoload.php';

The current file

Filesystem path – web server referenced

  echo ( __DIR__ );
  //Example output
  //  /home/websiteusername/public_html/the-directory-this-file-is-in

  echo ( __FILE__ );
  //Example output
  //  /home/websiteusername/public_html/the-directory-this-file-is-in/this-file-name.php