Most browsers (essentially all of them) should be able to detect a favicon without the help of any code. Just make sure it's called favicon.ico and is in the root directory of public_html/ or the root directory of your site. I'm not sure about any code to insert a favicon with PHP. You may be able to include a HTML file with the needed code, but I'm not sure if that will work.
Edit: I see you're using a PNG file for your favicon. If you wanted to try the auto-detection method mentioned above, you may have to convert it first to an ICO file (icon format). There are several sites on the Internet that allow you to do this online for free.
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