You can do it using DOMDocument (the best way):
$doc = new DOMDocument();
@$doc->loadHTML($html);
$imgs = $doc->getElementsbyTagName('img');
$res = $imgs->item(0)->getAttribute('src');
print_r($res);
With a regex (the bad way):
if (preg_match('~\bsrc\s*=\s*["\']\K[^"\']*+~i', $html, $match))
print_r($match);