<?php
$content = <<<'STR'
<img class="leftimg"
src="https://e...content-available-to-author-only...e.ru/img/shema/2016-05-20%2013-58-37.jpg"
alt="Забор из профнастила" width="550" height="309" />
STR;
$content = preg_replace('!<img(.*?)src(.*?)alt="(.*?)" width(.*?)height(.*?)\s\/>!si', '<span itemscope itemtype="https://s...content-available-to-author-only...a.org/ImageObject">
<span itemprop="name" style="display:none;">\\3</span>
<span itemprop="author" style="display:none;">Author</span>
<img itemprop="url contentUrl" \\1 src\\2 alt="\\3" title="\\3" width\\4 height\\5 />
<meta itemprop="width" content\\4>
<meta itemprop="height" content\\5></span>',$content);
echo $content;
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