如果从example.com链接显示内容

Is there any possibility to show html content on a site if the user was linked from a specific site? If not, do not show content.

I have a <input> login form and want to set value if the user comes from a special site.

Not too reliable but you can try:

<?php
if ($_SERVER['HTTP_REFERER']=="http://www.google.com") {
    echo "secret content";
}
?>