function convert_steamid_to_accountid($steamid)
     {
        $toks = explode(":", $steamid);
        $odd = (int)$toks[1];
        $halfAID = (int)$toks[2];

        $authid = ($halfAID*2) + $odd;
        echo $authid;
      }

<INPUT TYPE = "Text" VALUE ="ENTER STEAM:ID" NAME = "idform">

<?PHP
$_POST['idform'];
$steamid = $_POST['idform'];
?>

<form method="POST" action="foo.php">
  <input type="text" value="ENTER STEAM:ID" name="idform" />
  <input type="submit" />
</form>

<?php
  function convert_steamid_to_accountid($steamid)
  {
    $toks = explode(":", $steamid);
    $odd = (int)$toks[1];
    $halfAID = (int)$toks[2];

    $authid = ($halfAID*2) + $odd;
    echo $authid;
  }

  $id = $_POST['idform'];
  convert_steamid_to_accountid($id)
?>

if ($_SERVER['REQUEST_METHOD']=='POST'){
  // your php code here that should be executed when the form is submitted.
}