como guardar varios checkbox con php y mysql

[lextor04]

Holas...!soy nuevo y tengo un gran problema, intento por varios lados y pudo guadar los chekbox seleccionados en mi campo zona_a, este es mi codigo:

<table width="500" border="0" cellpadding="0" cellspacing="0">
<!--DWLayoutTable-->
<tr>
<td width="480" height="1"></td>
<td width="20"></td>
</tr>
<tr>
<td height="40" colspan="2" align="center" valign="middle" style="border:1px solid #CCCCCC"><strong class="texto">ZONAS TEMATICAS A RECORRER</strong></td>
</tr>
<tr>
<td height="2"></td>
<td></td>
</tr>

<tr>
<td height="24" align="left" valign="middle" style="border:1px solid #CCCCCC">&nbsp;A: Zona Costa:
<label></label></td>
<td valign="middle"><input type="checkbox" name="zona_a" id="zona_a" value="Zona Costa" onClick="zonas(this)"/></td>
</tr>

<tr>
<td height="2"></td>
<td></td>
</tr>

<tr>
<td height="24" align="left" valign="middle" style="border:1px solid #CCCCCC">&nbsp;B: Zona Sierra:</td>
<td valign="middle"><input type="checkbox" name="zona_a" id="zona_a" value="Zona Sierra" onClick="zonas(this)"/></td>
</tr>

<tr>
<td height="2"></td>
<td></td>
</tr>

<tr>
<td height="24" valign="middle" style="border:1px solid #CCCCCC">&nbsp;C: Zona Selva:</td>
<td valign="middle"><input type="checkbox" name="zona_a" id="zona_a" value="Zona Selva" onClick="zonas(this)"/></td>
</tr>

<tr>
<td height="2"></td>
<td></td>
</tr>

<tr>
<td height="24" valign="middle" style="border:1px solid #CCCCCC">&nbsp;D: Zona Internacional</td>
<td valign="top"><input type="checkbox" name="zona_a" id="zona_a" value="Zona Internacional" onClick="zonas(this)"/></td>
</tr>

<tr>
<td height="2"></td>
<td></td>
</tr>

<tr>
<td height="24" valign="middle" style="border:1px solid #CCCCCC">&nbsp;E: Zona Arqueol&oacute;gica</td>
<td valign="middle"><input type="checkbox" name="zona_a" id="zona_a" value="Zona Arqueologica" onClick="zonas(this)"/></td>
</tr>

<tr>
<td height="2"></td>
<td></td>
</tr>

<tr>
<td height="24" valign="middle" style="border:1px solid #CCCCCC">&nbsp;F: Zona Jardin Bot&aacute;nico</td>
<td valign="middle"><input type="checkbox" name="zona_a" id="zona_a" value="Zona Jardin Botanico" onClick="zonas(this)"/></td>
</tr>

<tr>
<td height="2"></td>
<td></td>
</tr>
<tr>
<td rowspan="2" valign="middle" style="border:1px solid #CCCCCC">&nbsp;G: Zona Cactario y Especies Nativas</td>
<td height="20" valign="middle"><input type="checkbox" name="zona_a" id="zona_a" value="Zona Cactario de Lima" onClick="zonas(this)"/></td>
</tr>
<tr>
<td height="1"></td>
</tr>


</table>