<?php
$url = 'http://t...content-available-to-author-only...m.br/15516553-medium.jpg?ver=1';
preg_match("/^.*(uol.com.br\/)(\d+)-*/", $url, $valores);
echo $valores[2]; // seu id ficará na posição [2]
PD9waHAKCiR1cmwgPSAnaHR0cDovL3QuLi5jb250ZW50LWF2YWlsYWJsZS10by1hdXRob3Itb25seS4uLm0uYnIvMTU1MTY1NTMtbWVkaXVtLmpwZz92ZXI9MSc7CnByZWdfbWF0Y2goIi9eLioodW9sLmNvbS5iclwvKShcZCspLSovIiwgJHVybCwgJHZhbG9yZXMpOwoKZWNobyAkdmFsb3Jlc1syXTsgLy8gc2V1IGlkIGZpY2Fyw6EgbmEgcG9zacOnw6NvIFsyXQo=