<?php

$urls = [
    "https://m...content-available-to-author-only...d.pt/sdashdasda",
    "https://m...content-available-to-author-only...o.nz/hfghfghf",
    "https://o...content-available-to-author-only...r.com/1gfdg5d4",
];

foreach($urls as $url) {
    echo "https://p...content-available-to-author-only...r.com/?url=" . base64_encode($url), PHP_EOL;
}