<?php

define('DOC_ROOT', '/foo/');
$string = '<img src="anything.jpg"> <img src="foobar.jpg"> <img src="/already/a/relative/path.jpg">';

$string = preg_replace('/<img([^>]*)src=["\']["\'\\/]([^"\']*)["\']/', '<img\1src="'.DOC_ROOT.'\2"', $string);
echo $string;

<img src="anything.jpg"> <img src="foobar.jpg"> <img src="/foo/already/a/relative/path.jpg">