Modification attribut alt dans un contenu

<?php

function        img_action(SimpleXMLElement &$XmlNode)
{
  foreach ($XmlNode->attributes() as $name => $value)
    {
      if ($name == 'alt' && $value == '')
        {
          $XmlNode['img'] = 'bidule';
          return ;
        }
      elseif ($name == 'alt')
        return ;
    }
  $XmlNode->addAttribute('alt', 'truc');
}

function        edit_img(SimpleXMLElement $XmlNode)
{
  if (sizeof($XmlNode->children()) <= 0)
    return $XmlNode;
  foreach ($XmlNode->children() as $Child)
    {
      if ($Child->getName() == 'img')
        img_action($Child);
      else
        edit_img($Child);
    }
  return $XmlNode;
}

$str = '                                                                        

<div>                                                                           
  <p><img src="lipsum.png" alt="lipsum" />Lorem ipsum dolor sit amet.</p>       
  <img src="toto.png" alt="" />                                                 
  <p>Sed ut perspiciatis unde omnis iste natus error <img src="sed.png" /></p>  
</div>';

$DataXml = new SimpleXMLElement($str, null);
$DataXml = edit_img($DataXml);
echo $DataXml->asXML(), "\n";