PHP: Faking “typecasting” with reflection

As far as type systems go, PHPs is pretty schizophrenic. You’ve got primitive types, like strings and booleans, the ubiquitous “array” type, and then user defined classes. Most of the time, the type system is invisible since it barely enforces anything. Especially for basic types and the standard library, you can almost always use strings, booleans, and integers interchangeably without much complaining from the interpreter. Where things go sideways is when you start using user defined types, especially with type hinting.

Imagine we’ve got the following setup:

If you run that in a terminal, PHP will throw the following error:

PHP Catchable fatal error: Argument 1 passed to sayHello() must be an instance of Dog, instance of Pet given, called in /home/ashish/Downloads/dog.php on line 19 and defined in /home/ashish/Downloads/dog.php on line 14

Because even though every “Dog” is by definition a superset of the “Pet” class, PHP doesn’t see it that way. And now, our original problem. In most other object oriented languages, you’d be able to simply typecast the instance of Pet to a Dog and then call the function as expected. Unfortunately, PHP doesn’t natively support typecasting so we’re stuck looking for a crazy workaround. Enter Reflection. PHPs reflection library lets you do all sorts of nefarious things, like manipulating private properties and retrieving the source for an arbitrary object.

So how do you use it to do a bootleg typecast? It’s actually pretty straightforward:

The “copyShimmedObject” is the money maker. It basically pulls the private properties out of the “from”, makes the property public, and then sets them on the “to” object. If you run the sample you’ll get the expected output instead of the error above:

Hello: Fluffy of destroyer of worlds

Posted In: PHP, Tips n' Tricks