var xmlhttp = getXmlHttp();
xmlhttp.open('GET', 'хост/test/dtp.php?feature_dtp='+feature_dtp, false);
xmlhttp.send(null);
if(xmlhttp.status == 200) {
var popup = xmlhttp.responseText;
};

var json_dtp = L.geoJson(dtpdata, {
       pointToLayer: function (feature, latlng) {
                var feature_dtp=encodeURIComponent(feature.properties.dtp);
                function getXmlHttp(){
                                      var xmlhttp;
                                      try {
                                           xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
                                      } catch (e) {
                                      try {
                                           xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
                                      } catch (E) {
                                      xmlhttp = false;
                                      }
                                     }
                                     if (!xmlhttp && typeof XMLHttpRequest!='undefined') {
                                            xmlhttp = new XMLHttpRequest();
                                     }
                                     return xmlhttp;
                                    };
                var xmlhttp = getXmlHttp();
                xmlhttp.open('GET', 'http://хост/test/dtp.php?feature_dtp='+feature_dtp, false);
                xmlhttp.send(null);
                if(xmlhttp.status == 200) {
                                           var popup = xmlhttp.responseText;
                };
                return L.marker(latlng, {title: feature.properties.dtp}).bindPopup(popup);
       },
       onEachFeature: dtp_onEachFeature
});

<script type="text/javascript"><!--
function ReplaceContentInContainer(id,content) {
var container = document.getElementById(id);
container.innerHTML = content;
}
//--></script>

<div 
   id="example1div" 
   style="border-style:solid; 
          padding:10px; 
          text-align:center;">
I will be replaced when you click.
</div>
<a href="javascript:ReplaceContentInContainer('example1div','Whew! You clicked!')">
Click me to replace the content in the container.
</a>