Most browsers won't reliably display RTF content. It IS possible to parse the RTF into HTML, and display the HTML content on your web page however.
You need some kind of program to parse RTF and convert it to HTML. I'm assuming it has to be free. I do not know of any reliable free RTF parsing or RTF to HTML libraries in PHP.
I recommend you use a command-line conversion program like RTF2HTML: http://sageshome.net/?w=downloads/soft/RTF2HTML.html
You would need to download and install this program on your webserver, allow the user to upload the file to a temp directory, and then call the command line application from PHP with shell_exec():
$html_output_path = '/path/for/processing/files/'
$html_output_filename = $username . $timestamp;
if (is_uploaded_file($_FILES['userfile']['tmp_name'])
{
shell_exec('rtf2html ' .
escapeshellarg($_FILES['userfile']['tmp_name']) . " " .
$html_output_path . $html_output_filename);
}
$html_to_display = file_get_contents($html_output_path .
$html_output_filename);
Then, parse the results as HTML and display them. Not a bad strategy. Note that you will probably need to remove the head, body and possibly other tags if you're going to display the content inside another web page.