<?php
$link_text = '<a href="http://w...content-available-to-author-only...e.com/mypage"><img src="imagelink"></a>';
$link_text = preg_replace('~(<a href=")(.*?)("><img src=")(.*?)("></a>)~i', '$1$4$3$4$5', $link_text);
print $link_text;
PD9waHAKCiRsaW5rX3RleHQgPSAnPGEgaHJlZj0iaHR0cDovL3cuLi5jb250ZW50LWF2YWlsYWJsZS10by1hdXRob3Itb25seS4uLmUuY29tL215cGFnZSI+PGltZyBzcmM9ImltYWdlbGluayI+PC9hPic7CgokbGlua190ZXh0ID0gcHJlZ19yZXBsYWNlKCd+KDxhIGhyZWY9IikoLio/KSgiPjxpbWcgc3JjPSIpKC4qPykoIj48L2E+KX5pJywgJyQxJDQkMyQ0JDUnLCAkbGlua190ZXh0KTsKCnByaW50ICRsaW5rX3RleHQ7