var username = "<?php echo $username ?>";
In the above code, we simply defined a new variable in JS and named it as username. We then assigned the PHP variable $username to that JS variable by echoing it. Naturally, the $username variable should be defined previously in your page, above this line.
Note that we used quotes (") around the PHP code that outputs the $username, since using quotes around strings is required when declaring a variable in JS. If the PHP output is a number, you don't need to use quotes. But make sure to use the quotes correctly in case the output string may also contain quotes inside it, hence break the JS code.
In the second method, the JS code is stored in an external JS file (e.g. script.js) and the file is linked from the HTML page. For example, if you want to pass a variable time value (in seconds) to a JS timer, you can use the following code:
var seconds = <?php echo $seconds ?>;
And something like this in your external JS file (script.js):
window.location.href = "/";
}, seconds * 1000);
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