最近在swoole中使用php pdo扩展访问数据库的时候，发现了一个很有意思的事情。

我测试用的版本是 PHP 7.1.13

官方手册明确到告诉我们，使用PDO是这样关闭数据库连接的，只需要将PDO对象置为null即可，这段代码就是手册里的

<?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(); } 

我实际测下来也的确如此。 这是query执行前的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) 

这是query运行后，PDO 对象置为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 对象置为null后，数据库连接的的确确关闭了，看来官方手册没错

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) 

但是接下来有意思的事情就要发生了，看这段代码

<?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对象被置为null之后，MySQL的连接并没有被真的释放

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) 

是不是很诡异？没关系，我们在改下这段代码

<?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$s也被置为null之后将$pdo置为null，pdo申请的数据库连接就被真真正正的释放了

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) 

这是为什么？看下面这段代码

$pdo = new PDO(     'mysql:dbname=test;host=127.0.0.1;port=3306;charset=utf8mb4', 'root', '',     [         PDO::ATTR_TIMEOUT => 10,         PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,         PDO::ATTR_PERSISTENT => false,         PDO::ATTR_EMULATE_PREPARES => false,         PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC     ] );  debug_zval_dump($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); 

输出

object(PDO)#1 (0) refcount(2){ } object(PDO)#1 (0) refcount(3){ } object(PDO)#1 (0) refcount(2){ } 

你会发现，当$pdo对象第一次打印的时候他的引用计数是2（为什么是2，而不是1呢？传送门 http://php.net/manual/zh/function.debug-zval-dump.php ）在$pdo->prepare之后引用计数变成了3，也就是说，即使$pdo被置为null，它依然还是会被$s引用，并不会被PHP的GC给回收掉，所以它创建的数据库连接也不会被释放。

在FPM模式下的PHP，这个问题并不会引起什么问题，但是在SWOOLE这种运行环境下，假如你的PDO对象在被一些全局变量（传送门 https://wiki.swoole.com/wiki/page/p-zend_mm.html ） 引用，即使你将PDO对象置为null，你的数据库连接依然不会被释放。