PHP function to verify whether the ID card is legal

/**
 *    身份证
 *
 *    @param    string    $id
 *    @return   boolean
 */
function is_idcard( $id )
{
    $id = strtoupper($id);
    $regx = "/(^\d{15}$)|(^\d{17}([0-9]|X)$)/";
    $arr_split = array();
    if(!preg_match($regx, $id))
    {
        return FALSE;
    }
    if(15==strlen($id)) //检查15位
    {
        $regx = "/^(\d{6})+(\d{2})+(\d{2})+(\d{2})+(\d{3})$/";
   
        @preg_match($regx, $id, $arr_split);
        //检查生日日期是否正确
        $dtm_birth = "19".$arr_split[2] . '/' . $arr_split[3]. '/' .$arr_split[4];
        if(!strtotime($dtm_birth))
        {
            return FALSE;
        } else {
            return TRUE;
        }
    }
    else           //检查18位
    {
        $regx = "/^(\d{6})+(\d{4})+(\d{2})+(\d{2})+(\d{3})([0-9]|X)$/";
        @preg_match($regx, $id, $arr_split);
        $dtm_birth = $arr_split[2] . '/' . $arr_split[3]. '/' .$arr_split[4];
        if(!strtotime($dtm_birth))  //检查生日日期是否正确
        {
            return FALSE;
        }
        else
        {
            //检验18位身份证的校验码是否正确。
            //校验位按照ISO 7064:1983.MOD 11-2的规定生成，X可以认为是数字10。
            $arr_int = array(7, 9, 10, 5, 8, 4, 2, 1, 6, 3, 7, 9, 10, 5, 8, 4, 2);
            $arr_ch = array('1', '0', 'X', '9', '8', '7', '6', '5', '4', '3', '2');
            $sign = 0;
            for ( $i = 0; $i < 17; $i++ )
            {
                $b = (int) $id{$i};
                $w = $arr_int[$i];
                $sign += $b * $w;
            }
            $n  = $sign % 11;
            $val_num = $arr_ch[$n];
            if ($val_num != substr($id,17, 1))
            {
                return FALSE;
            }
            else
            {
                return TRUE;
            }
        }
    }
   
}