<?php
$str = 'https://t...content-available-to-author-only...r.ru/1234/4321/';
echo $x['basename'], PHP_EOL;
PD9waHAKCiRzdHIgPSAnaHR0cHM6Ly90Li4uY29udGVudC1hdmFpbGFibGUtdG8tYXV0aG9yLW9ubHkuLi5yLnJ1LzEyMzQvNDMyMS8nOwokeCA9IHBhdGhpbmZvKCRzdHIpOwplY2hvICR4WydiYXNlbmFtZSddLCBQSFBfRU9MOwplY2hvIGJhc2VuYW1lKCR4WydkaXJuYW1lJ10pOw==