如何从PHP调用Ajax

Is it possible to run an ajax function from php like below? I know i can do it the other way but this would be the best way to do what i want i think.

Im trying to load a specific page based on some variables in php.

  <?php
     runFunction();
  ?>
  <script>
     function runFunction() {
        $.ajax({
           // do stuff?
        });
     }
  </script>

I have already tried the above and i've tried just putting the ajax in php... but everything i find on the old google just comes up php from ajax not the other way around.

thanks for any help.

You can try do it on JS event document.ready

<html>
  <script>
     function runFunction() {
        $.ajax({
           // do stuff?
        });
     }
  </script>
<?php
     echo "<body onload='runFunction()'></body>";
?>
</html>