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.

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which type of <input> creates input fields that should contain a numeric value?
type="text" type="number" type="date"
<input type="number" name="points" min="5" max="80" />
Which CSS property allows to change the transparency of an element?
font-style opacity color
#id {
  filter:alpha(opacity=40);    /* for IE */
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}
Click on the function that formats a number to specified number of characters.
toPrecision() exp() toFixed()
var num = 12.34567;
alert( num.toPrecision(3) );       // 12.3
Which function randomizes the order of the elements into an array?
natsort() shuffle() sort()
$lang =[10=>"PHP", 20=>"JavaScript", "site"=>"coursesweb.net");
shuffle($lang);
var_export($lang);     // array (0=>"coursesweb.net", 1=>"PHP", 2=>"JavaScript")
How to use php variable in external js file

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