validation of checkboxes and dropdowns together

[bale]

if (isset($_POST['submit']))

I want validation to see one of checkbox is chekced

and if checkbox is checked... then user should select a option from dropdown..

<table width="282">
         <tr> <td width="39%"> <input type="checkbox" name="hearaboutus[]" VALUE="Radio" <? if ($hearaboutus && in_array ("Radio", $hearaboutus)) echo ' checked="checked"'; ?>/>Radio </td>
         <td width="58%" >
         <?php
				$query = "SELECT source_detail FROM hearaboutus WHERE source = 'Radio'";
				$result = mysql_query($query, $db) or die(mysql_error());

			echo '<select name="Drophearaboutus1" style="width:150px;">';
			echo '<option value="" selected>'?> Select Station <? '</option> <br>';

			while($row = mysql_fetch_assoc($result)){
				$selected = $Drophearaboutus1 == $row['source_detail'] ? 'selected="selected"' : '';

				 echo '<option value="'.$row['source_detail'] .'"  '.$selected.' >'.$row['source_detail'] . '</option><br>';
			}
			echo '</select>';
	?>
     </td></tr>
     <tr><td> <input type="checkbox" name="hearaboutus[]" VALUE="Television" <? if ($hearaboutus && in_array ("Television", $hearaboutus)) echo ' checked="checked"'; ?>/>Television</td>
     <td>
     <?php
			$query = "SELECT source_detail FROM hearaboutus WHERE source = 'Television'";
			$result = mysql_query($query, $db) or die(mysql_error());

			echo '<select name="Drophearaboutus2" style="width:150px;">';
			echo '<option value="" selected>' ?> Select Channel <? '</option> <br>';
			while($row = mysql_fetch_assoc($result)){
				$selected = $Drophearaboutus2 == $row['source_detail'] ? 'selected="selected"' : '';
				 echo '<option value="'.$row['source_detail'] .'"  '.$selected.'>'.$row['source_detail'] . '</option><br>';
			}
			echo '</select>';
	?>
     </td></tr>
      <tr><td> <input type="checkbox" name="hearaboutus[]" VALUE="Print" <? if ($hearaboutus && in_array ("Print", $hearaboutus)) echo ' checked="checked"'; ?>/>Print</td>
     <td>
     <?php
			$query = "SELECT source_detail FROM hearaboutus WHERE source = 'Print'";
			$result = mysql_query($query, $db) or die(mysql_error());

			echo '<select name="Drophearaboutus3" style="width:150px;">';
			echo '<option value="" selected>' ?> Select Advertisement <? '</option> <br>';
			while($row = mysql_fetch_assoc($result)){
				$selected = $Drophearaboutus3 == $row['source_detail'] ? 'selected="selected"' : '';
				 echo '<option>'.$row['source_detail'] . '</option><br>';
			}
			echo '</select>';
	?>
     </td></tr>


   </table>

[NogDog]

One possible approach:

if(isset($_POST['checkbox_name'])) {
   if(empty($_POST['select_name'])) {
      // OK, continue processing
   }
   else {
      // handle error here
   }
}