PHP - hash objects in a way distint object with same fields values have same hash

Question

I am looking for a way to generate some kind of hash for PHP object (generic solution, working with all classed, built-in and custom, if possible).

SplObjectStorage::getHash is not what I'm looking for, as it will generate different hash for every instance of given class. To picture the problem, let's consider simple class:

class A() {
public $field; //public only for simplicity
}

and 2 instances of that class:

$a = new A(); $a->field = 'b';
$b = new A(); $b->field = 'b';

Every built-in function I've tried will return different hashes for these objects, while I'd like to have some function f($x) with property f($a) == f($b) => $a == $b.

I am aware I could write a function traversing all object's properties recursively until I find a property that can be casted to string, concatenate this strings in fancy way and hash, but the performance of such solution would be awful.

Is there an efficient way to do this?

Answer 1

Assuming I understand you correctly, you could serialize the objects then md5 the serialized object. Since the serialization creates the same string if all properties are the same, you should get the same hash every time. Unless your object has some kind of timestamp property. Example:

class A {
    public $field;
}
$a = new A;
$b = new A;
$a->field = 'test';
$b->field = 'test';
echo md5(serialize($a)) . "\n";
echo md5(serialize($b)) . "\n";

output:

0a0a68371e44a55cfdeabb04e61b70f7
0a0a68371e44a55cfdeabb04e61b70f7

Yours are coming out differently because the object in php memory is stored with a numbered id of each instantiation:

object(A)#1 (1) {...
object(A)#2 (1) {...