¿Cómo perdón por no saludar, si saludaste ;)?
Código PHP:
<?
$select_data = mysql_db_query('my_db', "SELECT category FROM theTable WHERE id='1'")
or trigger_error(mysql_error(),E_USER_ERROR);
$data = mysql_fetch_assoc($select_data);
$options = explode(",",$data);
?>
<select name="category[]" multiple="muliple" rows="3">
<option value="1"<?php if (in_array("1",$options)) { echo " selected=\"selected\""; } ?>>Acción</option>
<option value="2" <?php if (in_array("2",$options)) { echo " selected=\"selected\""; } ?>>Ciencia Ficción</option>
<option value="3"<?php if (in_array("3",$options)) { echo " selected=\"selected\""; } ?>>Aventura</option>
<option value="4"<?php if (in_array("4",$options)) { echo " selected=\"selected\""; } ?>>Drama</option>
<option value="5"<?php if (in_array("5",$options)) { echo " selected=\"selected\""; } ?>>Infantil</option>
<option value="6"<?php if (in_array("6",$options)) { echo " selected=\"selected\""; } ?>>Clásica</option>
</select>