<form action="index.php" method="post">
<input type="text" name="user_name" />
<input type="submit" value="Отправить" />
<input type="hidden" name="form_id" value="1" />
</form>
<form action="index.php" method="post">
<input type="text" name="user_name" />
<input type="submit" value="Отправить" />
<input type="hidden" name="form_id" value="2" />
</form>
<form action="index.php" method="post">
<input type="text" name="user_name" />
<input type="submit" value="Отправить" />
<input type="hidden" name="form_id" value="3" />
</form>
<?php
$formId = isset( $_POST['form_id'] ) ? $_POST['form_id'] : 1;
// $formId - наш идентификатор формы =)
?>