Calling PHP file using Ajax
But the second version has problems also because the obj variable is in the global namespace. It might be a good idea to abstract this into a function which contains a reference to the XMLHttpRequest object, and avoids polluting globals as in the first example.

var CurrentTime = function() {
    this.requestObj = (window.XMLHttpRequest) ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");
    this.element = document.getElementById("current_time");
};

CurrentTime.prototype.get = function() {
    var self = this;
    this.requestObj.open("GET", "current_time.php", true);
    this.requestObj.send(null);                            
    this.requestObj.onreadystatechange = function() {
        if (self.requestObj.readyState == 4 &amp;&amp; self.requestObj.status == 200) {
            self.element.innerHTML = self.requestObj.responseText;
        }                       
    };
};

var currentTime = new CurrentTime();
setInterval(currentTime.get.apply(currentTime), 1000);


You could also pass the location of the PHP file to this CurrentTime function for further abstraction, however I think this is good as is. The apply method is used to set the context of the method to the currentTime variable and not the window, so that you don't get TypeErrors. :)