获取实际打开的db连接

How can I check if a db-connection is open in PHP?

How can I get a reference to this connection?

How can I determine if it is MySQL, Postgres or something else?

Ideally you would know the variable the connection is saved into. Without this it becomes a hassle to find it. It means you would have to loop through every variable and test each one, but doing this for any reason can be a pain because it wont find variables outside of the current scope. You can loop the current variables using:

foreach(get_defined_vars() as $x){
    // Code
}

To check if a variable is a valid connection you can use the below code, it should return an integer:

if(mysql_thread_id($connection){
    // Connection exists
}

Once you know its a connection you can use the following to return the type of connection, eg, mysql.

echo get_resource_type($connection);

You'd have to iterate over every defined variable and figure out if it's a resource handle or not. Try get_defined_vars().

Probably the best solution to the problem is to not lose track of the DB connection in the first place.

Consider integrating an ORM into your project such as Doctrine or Propel, or if you want something a little more lightweight, consider a DBAL such as Zend_Db. All of these solutions encapsulate the database connections in class instances, ensuring that they are always available to your code if necessary.

In general terms, I don't think it's really possible, not at least with some sort of complicate workaround. You can, for instance, loop through all defined variables:

foreach(get_defined_vars() as $variable){
    if( is_resource($variable) && get_resource_type($variable)=='mysql link' ){
            // Found a connection!
        var_dump($variable);
    }
}

Though, in fact, a connection remains available even if assigned to a local variable out of current scope:

function foo(){
    $conn = mysql_connect('localhost', 'test', 'test');
}
foo();
$res = mysql_query('SELECT NOW() AS what_is_the_time');
if( $row = mysql_fetch_assoc($res) ){
    var_dump($row);
}

The problem is that PHP doesn't really care about variables when talking about connections:

// One connection and zero variables!
mysql_connect('localhost', 'test', 'test');
$res = mysql_query('SELECT NOW() AS what_is_the_time');
if( $row = mysql_fetch_assoc($res) ){
    var_dump($row);
}

You can try and find information about current connection:

var_dump(mysql_thread_id());
// int(2696)

... though many of the functions will just attempt to stablish a new connection if there isn't one so must take care of warnings...:

Warning: mysql_thread_id(): Access denied for user ''@'localhost' (using password: NO)
bool(false)

... unless your configuration actually allows a succesful credential-less connnection attempt.

I'm pretty sure that, whatever you want to do, can be done in an entirely different way :)