php xpath get node where attribute equals

Question

I have an xml how can I get the node in levelone that has an attribute called myatt whose value is a and then access it's myval. I tried referencing other posts to make it work but it doesn't seem to work what's wrong with my xpath

$this->myXmlObj->xpath("//levelone[myfield[attributes/myatt='a]]]"));

<myxml>
  <levelone>
      <myfield myatt="a" myval="aa" />
      <myfield myatt="b" myval="bb" />

  </levelone>  
  <leveltwo>
      <myfield myatt="c" myval="dd" />
      <myfield myatt="c" myval="dd" />
  </leveltwo>
</myxml>

edit 1

array
  0 => 
    object(SimpleXMLElement)[41]
      public '@attributes' => 
        array
          'myval' => string 'a' (length=40)

edit 2

    $myVar = $this->myXmlObj->xpath("//levelone/myfield[@myatt='a']");
    $myOutput = ((string)$myVar[0]->attributes()->myVal;

Answer 1

Attributes in XPATH are referenced with @attr syntax. So, you could retrieve aa with the following xpath

//levelone/myfield[@myatt='a']/@myval

Which means, grab all myfield elements that have attribute myatt equal to 'a'. Then, from those, select the value of their myval attributes. Note that this could be multiple results.

A handy place to test XPATH expressions is at http://chris.photobooks.com/xml/default.htm.