PHP: share database connections or create new one?

Question

I have been advice not to use the singleton pattern for my data base class. I have not fully understand what are the cons to it, anyway there seems to be agreement in this point so I follow the rule.

But, is sharing instances of classes a bad habit in PHP in general? Which is best from the two examples provide here? I am using in fact the singleton pattern with the first approach?

//semi-pseudo Code

Class DB extends mysqli{
    //
}

Class A {
    $db; //Of type Class DB, initialized in the constructor.

    //In some method

    //Should I do this, so sharing the data base connection?
    $b = new DB( $db );

    // OR

    // Should I instantiate a new instance?
    $newDb = new DB();
    $b = new B ($newDb);

}

Class B {
    $db;//Of type Class DB initialized in the constructor.

I have a Class DB being a data base extension..

I have Class A with a member of type DB.

Class A needs to create an instance of Class B, which in turn also has a member of type DB.

I'm using the Dependency Injection pattern so A should pass a DB instance to B.

Should I instantiate a new DB instance to pass to B, or can I just pass a reference to A's DB instance.

score 2 · Answer 1 · answered Feb 11 '12 at 09:48

2

There's nothing wrong with using the Singleton pattern in PHP, like any language, too much of anything likely indicates poor design.

That said, Singleton is well suited to database access. Has anyone justified why it's supposedly a bad idea?

The main argument against Singleton is difficulty to test because you can't mock them with unit test suites. Personally I try to avoid mock objects in my tests (especially w/ PHPUnit), so it usually doesn't matter.

answered Feb 11 '12 at 09:48

quickshiftin

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Here is the argument: http://stackoverflow.com/questions/4595964/who-needs-singletons/4596323#4596323 – David Casillas Feb 11 '12 at 09:58
Yes, thats the main arg, but you can put your singleton implementation behind an interface so you can mock it – Peter Porfy Feb 11 '12 at 10:01
There are ways around it, but I prefer the choice I mentioned, avoiding mocks in the first place, lol. – quickshiftin Feb 11 '12 at 10:23

score 0 · Answer 2 · answered Feb 11 '12 at 09:49

0

Should I instantiate a new DB instance to pass to B, or can I just pass a reference to A's DB instance.

Of course you have to pass a reference of A's DB instance to B class.

answered Feb 11 '12 at 09:49

Eugene Manuilov

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If your requirement is to share DB instance, you have to pass it as reference into B class. It doesn't matter what do you use Singleton or DI, this is only about how to get DB instance in A class. But to pass it from A to B should be performed only by ref. – Eugene Manuilov Feb 11 '12 at 10:09
Well with Singleton you don't have to pass the instance around at all. Most Singleton implementations provide a getInstance() method or similar, thus the DB::getInstance() method could be called exclusively from within instances of classes A & B, however the instance would actually be the same, that's how Singleton works. You can of course pass around the instance as well, but why bother w/ a Singleton. – quickshiftin Feb 11 '12 at 10:25
Thanks for introduction into Singleton life, but I've read GoF book and use Singleton a lot in my projects. The question for my answer is `Should I instantiate a new DB instance to pass to B, or can I just pass a reference to A's DB instance.`, this is why your comment is not relevant for my answer. – Eugene Manuilov Feb 11 '12 at 10:32

PHP: share database connections or create new one?

2 Answers2