php - Search for substrings of text in a node in a XML file -

i have php found in q&a forum queries xml file:

$doc = new domdocument;                                        // create new dom document  $doc->preservewhitespace = false;                              // set features  $doc->formatoutput = true;                                     // create indents on xml   $doc->load('i.xml');                                        // load file   $xpath = new domxpath($doc);  $query = '//users/user/firstname[.= "'.$_post["search"].'"]';         // xpath (starts root node)  $names = $xpath->query($query);                                 // list of matched elements    $output="";  foreach ($names  $node) {     $output.=$doc->savexml($node->parentnode)."\n";             // parent of  "<firstname>" element  (the entire "<user>" node , children) (maybe parent node directly using xpath)                                                                  // , use savexml() convert string    }   echo $output."<br>\n\n";                                      // result    echo "<hr><br><b>below view results  html content. &nbsp; (see page's html code):</b>\n<pre>".htmlspecialchars($output)."</pre>";  

the script search values of firstname nodes in xml document input post, , return parent node of nodefirstname, if post input value matches of node values.

this script works well, returns queries contain entire value of firstname node, , not work if search substring of node's text (e.g query potato return potato, query pot, not give me results potato).

so how result contains substring of node's text, instead of entire value ?