What is the difference between using readfile() and file_get_contents() to include external HTML files in PHP?

The main difference between using readfile() and file_get_contents() in PHP to include external HTML files is that readfile() outputs the file directly to the browser, while file_get_contents() reads the file into a string variable. If you want to include external HTML files in your PHP code and output them directly to the browser, you should use readfile(). However, if you need to manipulate the content of the file before outputting it, file_get_contents() is the better option.

// Using readfile() to include external HTML file and output it directly to the browser
readfile("external_file.html");