<?php
$file = !empty($_GET['f']) && is_string($_GET['f']) && ctype_print($_GET['f']) ? trim(str_replace('..', '', (string)$_GET['f']), '/') : '';
$allowedPaths = [
    'index.php',
    'parsedown.php',
    'src.php',
];
$maskStrings = [
];

if(empty($file) || !isset($_GET['f']))
    $file = $allowedPaths[0];

if(in_array($file, $allowedPaths, true)) {
    $file = file_get_contents($file);
    foreach($maskStrings as $string)
        $file = str_replace($string, '*****', $file);
    highlight_string($file);
}