why can`t close mysql_connect?

[cngrit0305]

hi,
I am a novice with php,today,I found can`t close mysql_connect.

I have a object "db_config.php"

class DbConfig{
private $db_url = 'localhost:3333';
private $db_user = 'root';
private $db_pwd = 'root';
private $db_name = 'xinxiwang';
function __get($property_name){
return $this->$property_name;
}

public function getConn(){
$link = mysql_connect($this->db_url,$this->db_user,$this->db_pwd)
or die("can not connect to Mysql server");
mysql_select_db($this->db_name,$link)
or die("can not found database " . $this->db_name );
return $link;
}
}

I will in a other php file include it "test.php"

require (db_config.php);

function fri_select($db,$pagenum,$counts){
$sql = "call friendlink_select($pagenum,$counts)";
$result = mysql_query($sql,$db);
mysql_close($db);
if($db){
echo "in function, con is alive;<br>";
}else{
echo "in function , con is dead";
}
if(isset($db)){
echo "in function,con is alive;<br>";
}else{
echo "in function , con is dead";
}
if(!empty($db)){
echo "in function,con is alive;<br>";
}else{
echo "in function , con is dead";
}
return $result;
}

$db = new DbConfig();
$dbs = $db->getConn();

$result = fri_select($dbs,1,6);
if($dbs){
echo "out function, con is alive;<br>";
}else{
echo "out function , con is dead";
}
if(isset($dbs)){
echo "out function,con is alive;<br>";
}else{
echo "out function , con is dead";
}
if(!empty($dbs)){
echo "out function,con is alive;<br>";
}else{
echo "out function , con is dead";
}

it will out put

in function,con is alive;
in function,con is alive;
in function,con is alive;
out function, con is alive;
out function,con is alive;
out function,con is alive;

why???I had close it in function!!!who can tell me??
thanks very much!!!