Using PHP readfile() to Output File Contents to Browser or Another File

In PHP, the readfile() function is a convenient way to read a file and send its contents directly to the browser or another output stream.

The function signature is:

int readfile ( string $filename [, bool $use_include_path = FALSE [, resource $context ]] )

It takes a filename as its first argument, returns the number of bytes read on success, and false on failure.

Consider a text file data.txt with the following content:

Hello, World!
I am learning PHP.

The following PHP script demonstrates how to output this file to the browser, setting appropriate HTTP headers for a download:

<?php
$file = 'data.txt'; // file path

if (file_exists($file)) {
    header('Content-Disposition: attachment; filename=' . basename($file)); // download file
    header('Content-type: text/plain'); // set MIME type
    readfile($file); // output file content
} else {
    echo "File does not exist.";
}
?>

The script first checks that the file exists, then sends headers to suggest a download and specifies the content type before calling readfile() to stream the file.

When executed, PHP outputs the contents of data.txt and prompts the user to save a file named data.txt.

The readfile() function can also be used to copy a file’s contents to another file. The example below shows this usage:

<?php
$sourceFile = 'data.txt'; // source file
$targetFile = 'output.txt'; // destination file

if (file_exists($sourceFile)) {
    readfile($sourceFile, $targetFile); // output to target file
} else {
    echo "File does not exist.";
}
?>

Running this script copies the content of data.txt into output.txt. In both cases, readfile() provides a simple method for file output, whether for downloading or copying.