PHP Database connection practice

Question

I have a script that connects to multiple databases (Oracle, MySQL and MSSQL), each database connection might not be used each time the script runs but all could be used in a single script execution. My question is, "Is it better to connect to all the databases once in the beginning of the script even though all the connections might not be used. Or is it better to connect to them as needed, the only catch is that I would need to have the connection call in a loop (so the database connection would be new for X amount of times in the loop).

Yeah Example Code #1:

// Connections at the beginning of the script
$dbh_oracle = connect2db();
$dbh_mysql  = connect2db();
$dbh_mssql  = connect2db();

for ($i=1; $i<=5; $i++) {
   // NOTE: might not use all the connections
   $rs = queryDb($query,$dbh_*); // $dbh can be any of the 3 connections
}

Yeah Example Code #2:

// Connections in the loop
for ($i=1; $i<=5; $i++) {
   // NOTE: Would use all the connections but connecting multiple times
   $dbh_oracle = connect2db();
   $dbh_mysql  = connect2db();
   $dbh_mssql  = connect2db();

   $rs_oracle = queryDb($query,$dbh_oracle);
   $rs_mysql  = queryDb($query,$dbh_mysql);
   $rs_mssql  = queryDb($query,$dbh_mssql);
}

now I know you could use a persistent connection but would that be one connection open for each database in the loop as well? Like mysql_pconnect(), mssql_pconnect() and adodb for Oracle persistent connection method. I know that persistent connection can also be resource hogs and as I'm looking for best performance/practice.

Here is a good post on why persistent connections could cause problems

Answer 1

The best performance/practice rule is simple: do connect to one database only.

As for the connects - try to implement some database access class. Which can connect automatically on demand.

Answer 2

Use a lazy connection wrapper class:

class Connection
{
    private $pdo;
    private $dsn;

    public __construct($dsn)
    {
        $this->dsn = $dsn;
    }

    public query($sql)
    {
        //the connection will get established here if it hasn't been already
        if (is_null($pdo))
            $this->pdo = new PDO($this->dsn);

        //use pdo to do a query here

    }
}

I hope you're already using PDO. If not, you should be. PDO is database independent. If you did this using procedural functions, you'd have to create a new class for each database type.

Anyways, this is just a skeleton (you'd want to add a $params option in query(), for example), but you should be able to get the idea. The connection is only attempted when you call query(). Constructing the object does not make a connection.

As an aside, consider using Doctrine. It has lazy connections and makes life easier in general.