How do I add PHP code/file to HTML(.html) files?

To add PHP code to an HTML file, you will need to use PHP tags. You can do this by enclosing your PHP code in <?php and ?> tags.

For example:

<!DOCTYPE html>
<html>
  <head>
    <title>My PHP Page</title>
  </head>
  <body>
    <?php
		echo "Hello, World!";
	?>
  </body>
</html>

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Note that you will also need to save the file with a .php extension, rather than .html, in order for the PHP code to be executed by the server.

Alternatively, you can include a PHP file in an HTML file using the include or require function. For example:

<!DOCTYPE html>
<html>
  <head>
    <title>My PHP Page</title>
  </head>
  <body>
    <?php include 'header.php'; ?>
    <p>Some content</p>
    <?php include 'footer.php'; ?>
  </body>
</html>

This will include the contents of the header.php and footer.php files at the respective locations in the HTML file.