How can I restrict class instances in PHP?

I'm relatively new to PHP (and to programming in general). Keep that in mind.

Here's the basic scenario. I am building a web-based mini-university.

There are two key classes: Lesson and LessonSeries.

• Lesson has a Series property, which stores a singular LessonSeries if that lesson belongs to a series. It may not.
• LessonSeries has a Lessons property, which will hold an array of Lesson objects..

Both classes have an 'Id' property, which is an integer and will be unique to their class. My database ensures there will not be two Lessons or two LessonSeries with the same Id.

In numerous pages throughout my website, Lesson objects are iterated and (if they have a series), they will display the series they belong to, and sometimes use some of the other LessonSeries methods and properties.

So here's the issue: When a Lesson is pulled from the Db, it constructs a LessonSeries to correspond to it. But I don't want there to exist 20 instances of the same LessonSeries. Certain methods trigger a db query that only needs to be executed once per LessonSeries. This could lead to exponentially more db activity than necessary if there were 20 instances of essentially the same series.

I'm new to programming patterns, but here's what I think I need:

• I want a registry of unique LessonSeries.
• I want each unique LessonSeries to be shared by all lessons that belong to it.
• Ideally, I want this functionality to be within the LessonSeries class, without having to have a second manager class, if it's at all possible.
• Basically, what I want is that, whenever a LessonSeries is constructed, the registry will be checked first for the existence of an the id. If so, what is returned is a reference to the existing LessonSeries. If it DOESN'T exist in the registry, then it is created as usual.

With this said, I have no idea how to make this happen in PHP.