Usually you can't add php code in external javascript (js) file. But, you can add and use php code in the page in which the external javascript file is included.
So, you can apply the following technique to use value of php variable in js file.

1. In the page in which the external javascript file is included, output the value of the php variable to a javascript variable:
<?php
//php code..
$some_var ='value';
?>
<!doctype html>
<html lang='en'>
<head>
<meta charset='utf-8' />
<title>Page Title</title>
<script>
var js_var ='<?php echo $some_var; ?>';
</script>
</head>
<body>
Page content..

<script src='external_file.js'></script>
</body>
</html>

Now, in the script from external_file.js we can use the js_var (it contains the value of the php $some_var).
2. For example, in external_file.js:

alert(js_var); // value

The javascript variable must be defined before the external js file is included.

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How to use php variable in external js file

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