PHP Performance – How to Speed Up XML Parsing Code

I'm developing a web application and we are using XML as a database and I'm using Linux as a platform and centos 6.5 as a server. Changing some value would make a lot a lot of changes in about six XML files.

So here's the problem: Pressing button save after editing, adding, deleting takes about one or one and half minutes or more! This is because my code has to update about six XML files, log files and the crontab file. I wonder if there's any thing similar to threads in PHP?

I have my functions and all code I want, I just want to know how php can be used in a new way to make performance better. Can you guide me even by links to read?

I don't know what's exactly this problem is called to search about it. I'm still a beginner.

here's the update function I'm using :

 public static function update($tableName, $fieldsToUpdate, $newValues, $value = NULL) {
        //Load XML file
        $fileName = G_APP_SCRIPTS_PATH . "/XMLS/xmls/" . $tableName . ".xml";
        $xml = new DOMDocument;
        $xml->load($fileName);
        //Load XSD file
        $xsdfileName = G_APP_SCRIPTS_PATH . "/XMLS/xsds/" . $tableName . ".xsd";
        $xsd = new DOMDocument;
        $xsd->load($xsdfileName);
        //Generate Xpath
        $path = c_XML::generateXPath($xml, $xsd, $fieldsToUpdate, $value);
        $xpath = new DOMXPath($xml);
        //Query nodes to update
        $nodes = $xpath->query($path);
//        print_r($nodes->item(0));
        //If number of new values < number of elements to update,means we have to update
        //all specified elements in the XML without any condition
        if (count($newValues) < $nodes->length) {
            for ($i = 0; $i < $nodes->length / count($newValues); $i++)
                $newValues = array_merge($newValues, $newValues);
        }
        //Iterate over the nodes to update
        for ($i = 0; $i < $nodes->length; $i++) {
            $oldNode = $nodes->item($i);
            for ($index = 0; $index < count($fieldsToUpdate); $index++) {
                //Find the associated index in the $fieldsToUpdate array
                if (strcmp($oldNode->nodeName, $fieldsToUpdate[$index]) === 0) {
                    break;
                }
            }
            //Create new node with the old node's name
            $newNode = $xml->createElement($oldNode->nodeName);
            //var_dump($newNode);
            //if the new value is an array,means we have to update an inner node
            if (is_array($newValues[$index])) {
                //go to the desired node in XSD and get its children elements,
                //to know their format
                $xsdxpath = new DOMXPath($xsd);
                $children = $xsdxpath->query("/xs:schema/xs:element/xs:complexType/xs:sequence/xs:element/xs:complexType/xs:sequence/xs:element[@name='" . $oldNode->nodeName . "']/xs:complexType/xs:sequence/xs:element");
                //convert the $chlidren node list to an array to pass the createNode function.
                for ($c = 0; $c < $children->length; $c++) {
                    $childrenArray[$c] = $children->item($c);
                }
                //create the desired node
                c_XML::createNode($xml, $xsdxpath, $newNode, $childrenArray, $newValues[$index]);
            }
            //If the new value is not an array
            else
                $newNode->nodeValue = $newValues[$index];
            //Replace old node with new node
            $parent = $oldNode->parentNode;
            $parent->replaceChild($newNode, $oldNode);
        }
        //Validate & save XML
        $isValid = c_XML::validateXMLtoXSD($xml, $xsdfileName);
        if ($isValid == 1) {
            $xml->save($fileName);
            return true;
        }
        else
            return false;
    }

and when user update a value I use this way in updating other xml files :

  private static function change_value_in_xml_file($old_value, $new_value) {
            $all_nodes = c_XML_h::select(tables::xmlfile, '*');
           $nodes = array();
            for ($i = 0; $i < count($all_nodes); $i++) {
               $nodes[$i] = array();
//just get the nodes i want
               $nodes[$i][0] = $all_nodes[$i][0];
               $nodes[$i][1] = $all_nodes[$i][3];
            }
            //find old value and replace it with new value
            foreach ($nodes as $node) {
                $newValues[0] = $new_value;

                if ($node[1] == $old_value) {
                    $id_to_update = $node[0];
                    $fieldsToSelect[0] = xmlfile:: value;
                    c_XML_h::update(tables::xmlfile, $fieldsToSelect, $newValues, $id_to_update);
                }

            }
        }