How to make a 2nd. combo to use the option choosed at the 1st combo?

Hi Experts!

How to make a 2nd. combo to use the option choosed at the 1st combo?

Accordingly to the code bellow, the way it's coded the 2nd. combo doesn't use the 1st. combo parameter.

<?php

....

    //----------Code start Combo (DSGER Despesa Hierárquica)--------------------------------------------
  $nivel=1;    // default parameter

 $sql1 = "select sb_dge, dge_info, substr(descricao,1,25) as descricao from dsger 
             where tipo=2 and nivel=".$nivel." order by descricao;";

    // Configurar o nivel:
     $query1 = mysql_query($sql1);
     while ( $results1[] = mysql_fetch_object ( $query1 ) );
     array_pop ( $results1 );
    //----------Final Combo 2-------------------------------------------------------------------------

....

<br/>
/////// 1st. Combo that furnish the parameter "nível" //////
Nivel Totalizador Despesa:
<select name="nivel">
    <option value="1">    1     </option>
    <option value="2">    2     </option>
</select>

<?php
   //After choosen 1st. parameter

    //----------Inicio Combo 2 (DSGER Despesa Hierárquica)--------------------------------------------

     echo "Nivel....".$nivel;

     $sql1 = "select sb_dge, dge_info, substr(descricao,1,25) as descricao from dsger 
             where tipo=2 and nivel=".$nivel." order by descricao;";

    // Configurar o nivel:
     $query1 = mysql_query($sql1);
     while ( $results1[] = mysql_fetch_object ( $query1 ) );
     array_pop ( $results1 );
    //----------Final Combo 2-------------------------------------------------------------------------
?>

<br/>
Despesa (H):
<select name="sb_dsg">
<?php 
     //----------Intend to present Combo 2 configured--------------------
     foreach ( $results1 as $option ) : ?>
          <option value="<?php echo $option->sb_dsg; ?>"><?php echo $option->descricao; ?></option>
<?php 
     endforeach; 
?>
</select> 

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What must be done ?

Thanks in advance!