Why the getElementsByTagName is not working in this example

Question 1

Is this what you after?

$htmlStr = '<td colspan=3>
             <p class=5tablebody>
                 <span style=\'position:relative;top:14.0pt\'>
                     <img width=300 height=220 src="forMerrin_files/image020.png">
                 </span>
             </p>
         </td>';

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

$paragraphs = $doc->getElementsByTagName('img');
var_dump($paragraphs->item(0)->getAttribute('src'));

Outputs:

string 'forMerrin_files/image020.png' (length=28)

Question 2

The second argument of DOMXpath::query() has to be a context node, you can not just use some HTML string. I suggest using DOMXpath::evaluate() anyway. The syntax of both methods is the same, but query() is limited to Xpath expressions that return a node list, evaluate() allows Xpath expressions that return scalars, too.

$html = <<<HTML
  <td colspan=3>
    <p class=5tablebody>
      <span style='position:relative;top:14.0pt'>
        <img width=300 height=220 src="forMerrin_files/image020.png">
      </span>
    </p>
  </td>
HTML;

$dom = new DOMDocument();
$dom->loadHtml($html);
$xpath = new DOMXpath($dom);

// for each td element
foreach ($xpath->evaluate('//td') as $cell) {
  // for each img inside a p
  foreach ($xpath->evaluate('.//p//img', $cell) as $img) {
    var_dump($img->getAttribute('src')); 
  }
}

Output: https://eval.in/147576

string(28) "forMerrin_files/image020.png"