How to call a JavaScript function from PHP?

How to call a JavaScript function from PHP?

<?php

  jsfunction();
  // or
  echo(jsfunction());
  // or
  // Anything else?

The following code is from xyz.html (on a button click) it calls a wait() in an external xyz.js. This wait() calls wait.php.

function wait() 
{
  xmlhttp=GetXmlHttpObject();
  var url="wait.php"; \
  xmlhttp.onreadystatechange=statechanged; 
  xmlhttp.open("GET", url, true); 
  xmlhttp.send(null);
} 

function statechanged()
{ 
  if(xmlhttp.readyState==4) {
       document.getElementById("txt").innerHTML=xmlhttp.responseText;
  }
}

and wait.php

<?php echo "<script> loadxml(); </script>"; 

where loadxml() calls code from another PHP file the same way.

The loadxml() is working fine otherwise, but it is not being called the way I want it.