Php download instead of showing in the browser

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PHP

I have a form PHP page which creates a simple txt file. it also has a button to download the same text file. However, when clicked it's showing the file and downloading it. I want to force download it and not show in the browser (Any Browser).

Thanks

What I have tried:

echo "<form method=\"get\" action=\"lics/$site.txt\">";
echo "<div>";
echo "</div>";
echo "<input class=\"submit-button\" type=\"submit\" value=\"Download\">";
echo "</form>";