$category = array(  
    0=>array('cat_id'=>1,'cat_name'=>'北京市','parent_id'=>0),
    1=>array('cat_id'=>2,'cat_name'=>'东城区','parent_id'=>1),
    2=>array('cat_id'=>3,'cat_name'=>'西城区','parent_id'=>1),
    3=>array('cat_id'=>4,'cat_name'=>'昌平区','parent_id'=>1),
    4=>array('cat_id'=>5,'cat_name'=>'河北省','parent_id'=>0),
    5=>array('cat_id'=>6,'cat_name'=>'张家口','parent_id'=>5),
    6=>array('cat_id'=>7,'cat_name'=>'南邵镇','parent_id'=>4),
); 
 
$list = categoryTree($category,0,0);
 
echo showSelect($list,0);
exit; 
 
/**
 * 数据数组设置层级.
 *
 * @param array  $category   数据数组.
 * @param string $parent_id  父级id.
 * @param string $level      层级id.
 *
 * @return array.
 */
function categoryTree($category, $parent_id = 0, $level = 0) {
    static $res = array();
    foreach($category as $v) { 
        if($v['parent_id'] == $parent_id) {
            $v['level'] = $level;  
            $res[] = $v;  
            categoryTree($category,$v['cat_id'],$level+1);  
        }  
    } 
    return $res;  
}
/**
 * 显示下拉框.
 *
 * @param array  $list    数据数组.
 * @param string $cat_id  选中id.
 *
 * @return string.
 */
function showSelect($list, $cat_id = 0){
    $str = '<select id="cat_id" name="cat_id"><option value="0">请选择</option>';
    foreach($list as $row) {
    	$select = '';
    	if($cat_id==$row['cat_id']){
    		$select = "selected";
    	}
    	$str .= '<option value="'.$row['cat_id'].'">'.str_repeat('    ', $row['level']).'|---'.$row['cat_name'].'</option>';
    }
    $str.='</select>';
    return $str;
}