測試用的版本是 PHP 7.1.13
官方手冊明確到告訴我們，使用PDO是這樣關閉數據庫連接的，只需要將PDO對象置爲null即可，這段代碼就是手冊裡的
<?php
try {
$dbh = new PDO('mysql:host=127.0.0.1;dbname=test', 'root', '');
foreach($dbh->query('SELECT * from test') as $row) {
print_r($row);
}
sleep(100);
$dbh = null;
} catch (PDOException $e) {
print "Error!: " . $e->getMessage() . "<br/>";
die();
}我實際測下來也的確如此。 這是query執行前的MySQL的PROCESSLIST
mysql> SHOW PROCESSLIST; +----+------+-----------------+------+---------+------+----------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+------+-----------------+------+---------+------+----------+------------------+ | 10 | root | localhost:56477 | NULL | Query | 0 | starting | SHOW PROCESSLIST | +----+------+-----------------+------+---------+------+----------+------------------+ 1 row in set (0.00 sec)
這是query運行後，PDO 對象置爲null前的MySQL的PROCESSLIST
mysql> SHOW PROCESSLIST; +----+------+-----------------+------+---------+------+----------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+------+-----------------+------+---------+------+----------+------------------+ | 10 | root | localhost:56477 | NULL | Query | 0 | starting | SHOW PROCESSLIST | | 14 | root | localhost:56572 | test | Sleep | 2 | | NULL | +----+------+-----------------+------+---------+------+----------+------------------+ 2 rows in set (0.00 sec)
PDO 對象置爲null後，數據庫連接的的確確關閉了，看來官方手冊沒錯
mysql> SHOW PROCESSLIST; +----+------+-----------------+------+---------+------+----------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+------+-----------------+------+---------+------+----------+------------------+ | 10 | root | localhost:56477 | NULL | Query | 0 | starting | SHOW PROCESSLIST | +----+------+-----------------+------+---------+------+----------+------------------+ 1 row in set (0.00 sec)
但是接下來有意思的事情就要發生了，看這段代碼
<?php
$pdo = new PDO(
'mysql:dbname=test;host=127.0.0.1;port=3306;charset=utf8mb4', 'root', '',
[
PDO::ATTR_TIMEOUT => 10,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
PDO::ATTR_PERSISTENT => false,
PDO::ATTR_EMULATE_PREPARES => false,
PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
]
);
$s = $pdo->prepare("select * from test WHERE id=:id");
$s->execute(['id' => 1]);
$s->fetchAll();
$pdo = null;
sleep(100);在PDO對象被置爲null之後，MySQL的連接並沒有被真的釋放
mysql> SHOW PROCESSLIST; +----+------+-----------------+------+---------+------+----------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+------+-----------------+------+---------+------+----------+------------------+ | 10 | root | localhost:56477 | NULL | Query | 0 | starting | SHOW PROCESSLIST | | 18 | root | localhost:56606 | test | Sleep | 3 | | NULL | +----+------+-----------------+------+---------+------+----------+------------------+ 2 rows in set (0.00 sec)
是不是很詭異？沒關係，我們在改下這段代碼
<?php
$pdo = new PDO(
'mysql:dbname=test;host=127.0.0.1;port=3306;charset=utf8mb4', 'root', '',
[
PDO::ATTR_TIMEOUT => 10,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
PDO::ATTR_PERSISTENT => false,
PDO::ATTR_EMULATE_PREPARES => false,
PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
]
);
$s = $pdo->prepare("select * from test WHERE id=:id");
$s->execute(['id' => 1]);
$s->fetchAll();
$s=null;
$pdo = null;
sleep(100);在$s也被置爲null之後將$pdo置爲null，pdo申請的數據庫連接就被真真正正的釋放了
mysql> SHOW PROCESSLIST; +----+------+-----------------+------+---------+------+----------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+------+-----------------+------+---------+------+----------+------------------+ | 10 | root | localhost:56477 | NULL | Query | 0 | starting | SHOW PROCESSLIST | +----+------+-----------------+------+---------+------+----------+------------------+ 1 row in set (0.00 sec)
這是爲什麽？看下面這段代碼
$pdo = new PDO(
'mysql:dbname=test;host=127.0.0.1;port=3306;charset=utf8mb4', 'root', '',
[
PDO::ATTR_TIMEOUT => 10,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
PDO::ATTR_PERSISTENT => false,
PDO::ATTR_EMULATE_PREPARES => false,
PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
]
);
debug_zval_dump($pdo);
$s = $pdo->prepare("select * from test WHERE id=:id");
debug_zval_dump($pdo);
$s->execute(['id' => 1]);
$s->fetchAll();
$s=null;
debug_zval_dump($pdo);
$pdo = null;
sleep(100);輸出
object(PDO)#1 (0) refcount(2){
}object(PDO)#1 (0) refcount(3){
}object(PDO)#1 (0) refcount(2){
}你會發現，當$pdo對象第一次列印的時候他的引用計數是2（爲什麽是2，而不是1呢？傳送門 http://php.net/manual/zh/function.debug-zval-dump.php ）在$pdo->prepare之後引用計數變成了3，也就是說，即使$pdo被置爲null，它依然還是會被$s引用，並不會被PHP的GC給回收掉，所以它創建的數據庫連接也不會被釋放。
在FPM模式下的PHP，這個問題並不會引起什麽問題，但是在SWOOLE這種運行環境下，假如你的PDO對象在被一些全局變量（傳送門 https://wiki.swoole.com/wiki/page/p-zend_mm.html ） 引用，即使你將PDO對象置爲null，你的數據庫連接依然不會被釋放。
