Including external JavaScript in PHP file and calling a function

I have a PHP file in which I have a <script> tag where I defined chartIt() function to draw a chart.js graph. I need to use chartIt() function in another file so I want to move whole code from <script> tag to external JavaScript file but when I do that i got an error. My take:

<script type="text/javascript" src="myjavascript.js"></script>

if(!empty($_POST['btn'])){ 
        switch($_POST['btn']){
            case '1':
                ?>
    <script>
    chartIt(x_amp, 'X axis', "Red");
    </script>
    <?php
                break;

Errors I got:

Uncaught SyntaxError: Unexpected token '<'

index.php:121 Uncaught ReferenceError: chartIt is not defined at index.php:121

source https://stackoverflow.com/questions/67827376/including-external-javascript-in-php-file-and-calling-a-function