script.php

<?php
function rmNonAlpha($input) {
  return preg_replace("@[^0-9a-zA-Z\s]@", "", $input);
}

if(isset($_GET['filepath'])){
    $base_filepath = ""; // Enter here your base filepath (The directory where you want people to be able to read/write files). You should end it with an "/"
    $sanitized_filepath = rmNonAlpha($_GET['filepath']); // Removes non Alpha chars, so stuff like "/" will be ignored, so people won't leave directory
    $full_path = $base_filepath . $sanitized_filepath;
    if(file_exists($full_path)) {
        echo file_get_contents($path);
    } else echo "File doesn't exist";
} else echo "Filepath Missing";

?>