Simple form:

  <form action="/somepage.php" method="POST">
    <input type="hidden" name="Action" value="UpdateSettingsABC" />

    <p>
    <label>Enter Here</label>
    <input type="text" name="Text1" size="40"  />
    </p>

    <input type="submit" value="Send" id="Send" />
  </form>

Submit to same page, see here

onSubmit

E.g.

<form action="join.php" method="post" name="capform" class="hover_inputs" onSubmit="return capCheck(this);">

onSubmit=”return capCheck(this);” means call the javascript function called capCheck

 <script type="text/javascript" id="clientEventHandlersJS" language="javascript">
 	function capCheck(theForm) {
	}
  </script>

PHP Handle Form

  //------------------------
  //----- SAVE CHANGES -----
  //------------------------
  if ( (isset($_POST['Action'])) && ($_POST['Action'] == "UpdateSettings") )
  {
    $AdvertCreated = mysqli_real_escape_string($dblink, $_POST['AdvertExpires']);
    $TerminalPhoneNumber = intval($_POST['AdvertCreated']);


  }