accessing variables in functions / classes

[bampot]

i'm currently trying to write some classes however im having problems passing variables

the display_cat_menu calls the get_categories function, both are within the same class

<?php
  function get_categories()
  {
    // query database for a list of categories
    $conn = new db();
    $conn -> db_conn();
    $id = 'catid';
    $query = 'select $id, catname
              from categories';
    $result = @mysql_query($query);
    if (!$result)
      return false;
    $num_cats = @mysql_num_rows($result);
    if ($num_cats ==0)
      return false;

$result= $conn->db_result_to_array($result);
return $result;
  }


  function display_category_menu()
  {
?>
    <form method=get action="show_cat.php">
        <select name=catid>
        <?php
            // list of possible categories comes from database
            $cat_array=$this->get_categories();
            foreach ($cat_array as $thiscat)
            {
                 echo '<option value="';
                 echo $thiscat['catid'];
                 echo '"';
                 echo '>'; 
                 echo $thiscat['catname'];
                 echo "\n"; 
            }
       ?>
       </select>
    <input class="go" type=submit value="Go">
    </form>
<?php
  }

i have tried this query harcoded which works fine, but many attempts to pass the variable $id have not worked, ive tried passing through the function, using set and get functions etc. Could anyone point me in the right direction

Thanks in advance

[bampot]

I was able to get it to work using double quotes on the sql query and passing the variables via the functions. As this is in a class would it be better to practice to use a get/set function for these variables?

<?php
  function get_categories($id, $name, $table)
  {
    // query database for a list of categories
    $conn = new db();
    $conn -> db_conn();
    $query = "select $id, $name
              from $table";
    $result = @mysql_query($query);
    if (!$result)
      return false;
    $num_cats = @mysql_num_rows($result);
    if ($num_cats ==0)
      return false;

$result= $conn->db_result_to_array($result);
return $result;
  }

  function display_category_menu($id, $name, $table)
  {
?>
    <form method=get action="show_cat.php">
     <p id="menu">Theme:
        <select name=catid>
        <?php
            // list of possible categories comes from database
            $cat_array=$this->get_categories($id, $name, $table);
            foreach ($cat_array as $thiscat)
            {
                 echo '<option value="';
                 echo $thiscat['catid'];
                 echo '"';
                 echo '>'; 
                 echo $thiscat['catname'];
                 echo "\n"; 
            }
       ?>
       </select>
    <input class="go" type=submit value="Go">
    </form>
<?php
  }