Why does $message return as empty string ?

message.php

<?php

$message = file_get_contents(__DIR__ . '/confirmation.html');

$replace = array(
        '[site_url]' => get_site_url() . '/wp-content/themes/artists-working/img/email/',
        '[email]' => $email,
        '[event_id]' => $event_id,
        '[address]' => $event_address
);

foreach ($replace as $placeholder => $info) {
        $message = str_replace($placeholder, $message, $info);
}
echo var_dump($message); // returns "" ?!?!
?>