关闭连接时出现Mysql错误

Question

I have this class for DB interactions. It works fine.

<?php require_once('inc/config.inc.php'); ?>
<?php
final class MySQL {
private $link;

public function __construct($hostname, $username, $password, $database) {
    if (!$this->link = mysql_connect($hostname, $username, $password)) {
        //error('Error: Could not make a database link using ' . $username . '@' . $hostname);

    }
    if (!mysql_select_db($database, $this->link)) {
        //error('Error: Could not connect to database ' . $database);
    }
    //echo "open". time();
}

public function __destruct() {
    mysql_close(); // Problem In here
    //echo "closed". time();
}
}

$database=new MySQL(DB_SERVER,DB_USER,DB_PASS,DB_NAME);
?>

But if I replace mysql_close(); with mysql_close($this->link);

I get an error: Warning: mysql_close() expects parameter 1 to be resource, null given

Answer 1

The reason for this error is quite simple: $this->link is a resource. And (quoting the doc itself)...

... a resource with no more references to it is detected automatically, and it is freed by the garbage collector. For this reason, it is rarely necessary to free the memory manually.

In other words, it's closed before your __destruct method is called. Even more, in mysql_close documentation it's said:

Using mysql_close() isn't usually necessary, as non-persistent open links are automatically closed at the end of the script's execution.

By the way, I usually think of __destruct as of internal method (and quite rarely find use for it): PHP is not C++, the whole concept of freeing resources is rather different here (by a plenty of reasons). If I need to perform some cleanup task, I usually use register_shutdown_function, as it turned out to be way more predictable.