Help me about authentication.

[quocphong]

I have a script pHp :

<?php
if(!isset($PHP_AUTH_USER))
{
Header("WWW-Authenticate:Basic realm=\"MyRealm\"");
Header("HTTP/1.1 401 Unauthorized");
echo "Text to send if user hits Cancel button\n";
exit;
}
else
{
echo "Hello $PHP_AUTH_USER.<P>";
echo "You enter $PHP_AUTH_PW as your password.<p>";
}
?>

But when I access to it,the server(apache)output this error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, [e-mail] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

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Apache/1.3.20 Server at mutimedia32.phuongnam.com Port 80

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Please tell me how to repair it.Thankyou..