creating the file action.php

Good Day!

I have created a form for adding an announcement on the site

<form action="action.php" method="post">

<fieldset>

Pseudonym:<br><input name="pseudo" size="6" type="text"><br>Title:<br><input name="titre" size="6" type="text"><br>Message:<br>

<textarea cols="50" height="150" name="mess" rows="5" width="300">  </textarea><br>Photo:<br><input name="phot" size="6" type="text"><br>Phone:<br>

<input name="tel" size="6" type="text"><br>Email:<br>

<input name="email" size="6" type="text"><br>

<input type="submit" value="Publish">

</fieldset>

</form><p>&nbsp;</p>

I want to know where to save the file action.php in Drupal?

Thank you in advance!