how to run php function without reloading the page

how to run php function without reloading the page

I am a newbie to php
<?php
getDBData(){
//log the call
$fetchedData = myDbCode.fetchData();
return
}
?>
<script type="text/javascript">
dbData = <?php echo json_encode(getDBData()); ?>
</script>
As observed in the log that getDBData get called only once during the page
loading and later on even with dbData = <?php echo
json_encode(getDBData()); ?> this code the call to getDBData() doesn't
happen.
Any idea why the call to getDBData() happening only on page load and not
thenafter
How to call getDBData() from javascript