PHP中MySQL连接管理
需要注意的是:使用mysql_connect()获取共享MySQL连接资源时,调用mysql_close()有时并不能释放MySQL连接,只有当所有共享连接资
对于PHP程序来来说,MySQL是一种稀缺的资源。MySQL默认最大连接数为100,也即是最多接受100个MySQL的客户端连接。因此,PHP在使用MySQL连接时做了一些优化的处理。这些优化,可能对性能有所提升,但对使用MySQL连接的使用却也造成一些困惑。
创建MySQL连接:
resource mysql_connect ( [string server [, string username [, string password [, bool new_link [, int client_flags]]]]] )
当使用mysql_connect()创建一个MySQL连接后,PHP会把这个连接的信息,包括host, username, password与连接资源存储起来。下次调用mysql_connect()时,PHP会默认检查是否存在一个参数相同(参数包括host, username, password)的连接资源,如果存在,则直接返回之前创建的资源,而非新建一个MySQL连接。(PS:当两次调用连接的不是相同服务器时,只能创建新的MySQL连接,旧的连接无法共享)
只有当手动指定new_link为true时,PHP才会避开这个检查,直接重新向PHP服务器发起MySQL连接请求。
这种处理在下面这种情况下就会产生错误:
用户期望在database1查询table1的信息,但是由于在第二次mysql_connect(),PHP返回的是上一次的MySQL连接资源。因此,对于$conn2中切换数据库也对$conn1造成了影响,$conn1也同时切换默认到数据库database2,查询的信息会是在database2上面的结果。
解决这个问题的方法就是在第二次调用mysql_connect()时,指定new_link为true即可。
关闭MySQL连接:
对于使用mysql_connect()创建的连接资源,,对资源变量进行unset()并不能真正地释放资源,需要调用mysql_close()才能达到释放MySQL连接资源的效果。
需要注意的是:使用mysql_connect()获取共享MySQL连接资源时,调用mysql_close()有时并不能释放MySQL连接,只有当所有共享连接资源都调用mysql_close()时,PHP才会真正释放MySQL连接。

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