在您的foreach循环中，调用 $ img-> setAttribute（'class'，'someclass'）; 。这应该可以解决问题。在 http://docs.php.net/manual/en/domelement中了解更多信息。 setattribute.php

然后，您需要使用 $ article_header = $ doc-> saveXml（）;保存修改过的文档。 。

Here is piece of code

$doc = new DOMDocument();
$doc->loadHTML($article_header);

$imgs = $doc->getElementsByTagName('img');
foreach ($imgs as $img) {

Into $imgs goes DOM img tag. Now I want to change the original img tag by adding some class to it.

SO if the $article_header was this:

"some text"...<img src = 'http://some_source'>...some text...

Now I want it to become this:

"some text"...<img class = 'someclass' src = 'http://some_source'>...some text...

UPDATE

I repeat. Start variable is $article_header. So all changes must be done to it.

With my code I just search through $article_header for img tags , finding them putting them into some variables and change them there is ok, but how can I put all changes back to $article_header ???

In your foreach loop, call $img->setAttribute('class', 'someclass');. This should do the trick. See more at http://docs.php.net/manual/en/domelement.setattribute.php

Then you need to save the modified document back using $article_header = $doc->saveXml();.