How to call a PHP function on the click of a button

Question

The two buttons on my functioncalling.php page are labeled Submit and Insert.

I want to see which action a button takes when it is pressed. The output must show up on the same page, please. For each button, I, therefore, wrote two routines.

<form action="functioncalling.php">
    <input type="text" name="txt" />
    <input type="submit" name="insert" value="insert" onclick="insert()" />
    <input type="submit" name="select" value="select" onclick="select()" />
</form>

<?php
    function select(){
        echo "The select function is called.";
    }
    function insert(){
        echo "The insert function is called.";
    }
?>

The issue here is that after clicking any of the buttons, I receive no output.

What exactly am I doing incorrectly?