hidden INPUT value not available in $_POST

javascript php html post

PHP Snippet 1:

<form id="myForm" action="/post" method="post">
</form>

<input type="hidden" name="hidden-item" value="hidden-value"    
  **form="myForm"** />

PHP Snippet 2:

 <form action="server_side_code.php" method="post">
     <input type="text" name="value1" />
     <input type="hidden" name="value2" /> <!-- OK -->
     <input type="submit" />
 </form>

 <input type="hidden" name="value3" /> <!-- Will not be sent to anywhere -->

PHP Snippet 3:

<input type="hidden" name="first_name" value="png" disabled> // can not post to form

<input type="hidden" name="first_name" value="png">