I have .php function call activeNow(); and button for it.

Code:
at php script :
<?php
public function activeNow() {
   echo 'hello world';
}
?>


at design script :
<button onclick = "call_now();" >Active !!</button>

at javascript : 
<script>
function call_now() {
}
</script>
my question is, what should I must write into javascript if I press the Active!! button, then It will call activeNow() from php script ??

thanks