Just as someone can call a JS function on html, can it be done on php?

Question

Calling JS functions

Answer 1

Do you mean call a JS fonction through PHP ? If that well you simply have to echo your JavaScript.
ex:
<script>

//some Js before

var test = $("#input").val();
var feedback = function(fb) { $("error").text(fb) };

<?php
echo '
if(test.length === 0){

feedback("empty input !");

}';
?>

//some Js after

</script>

Answer 2

only pointing out that it is considered bad practice.