how to include js file in php?

I found a different solution that I like:

<script>
    <?php require_once("/path/to/file.js");?>
</script>

Also works with style-tags and .css-files in the same way.

PHP is completely irrelevant for what you are doing. The generated HTML is what counts.

In your case, you are missing the src attribute. Use

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

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