php hide menu

I have three drop down menus in my form. How do I make it so the second and third menus are only visible once the prior menu was selected? Below is the first two drop down menus. Thanks in advance. // Generating first menu using array. $markets = array('MCI' => 'Kansas City', 'STL' => 'ST. Louis', 'ICT' => 'Wichita', 'OMA' => 'Omaha', 'LIN' => 'Lincoln'); echo "<select name='term'><option value=''>Choose Market</option>\n"; foreach ($markets as $key => $market) { echo "<option value='$key'>$market</option>\n"; } echo "</select>"; // This will evaluate to TRUE so the text will be printed. if (isset($markets)) { echo "This var is set so I will print."; } // Then, later, validating the menu if (! array_key_exists($_POST['Market'], $choices)) { echo "You must select a market."; } $query="SELECT cell_sect FROM sector_list order by cell_sect"; $result = mysql_query ($query); echo "<select name='cat'><option value=''>Choose Cell Sector</option>"; // printing the list box select command while($cellSect=mysql_fetch_array($result)){//Array or records stored in $cellSect echo "<option value=$cellSect[cell_sect]>$cellSect[cell_sect]</option>"; /* Option values are added by looping through the array */ } echo "</select>";// Closing of list box // This will evaluate to TRUE so the text will be printed. if (isset($cellSect)) { echo "This var is set so I will print."; }