How does PHP use the JsonSerializable interface?

Question

I'm confused with how JsonSerializable works. Let me elaborate bit more the question.

Normally we use interfaces like this:

<?php
interface Countable {
    /* Methods */
    public function count();    
}

class MyThings implements Countable 
{
    public function count() {
        return count($this->arrayOfThings);
    }
}


$obj = new MyThings();

//call count method on $obj
$obj->count();

So we have a class and it implements the interface. When we call count() function, it is already written in MyThings class. It is simple to understand.

But when we use JsonSerializable interface like this:

<?php
class Thing implements JsonSerializable {
    public function jsonSerialize() {
        // do something 
    }
}
$obj = new Thing();

//call count method on $obj
json_encode($obj);

jsonSerialize() inside Thing runs with the json_encode() call. It is understandable if we call

$obj->jsonSerialize();

then there is a function called jsonSerialize() inside the class. But, how does this work when we run json_encode()? How this is constructed in php? Can someone explain what type of patterns used here?

Answer 1

Objects that implement JsonSerializable then implement a jsonSerialize() method. Then, when json_encode() is serializing its input into JSON, if a value that it is serializing is a JsonSerializable, it calls the jsonSerialize() method, and the result of that method is used as the object's serialized representation.

For example, from the PHP documentation:

<?php
class IntegerValue implements JsonSerializable {
    public function __construct($number) {
        $this->number = (integer) $number;
    }

    public function jsonSerialize() {
        return $this->number;
    }
}

echo json_encode(new IntegerValue(1), JSON_PRETTY_PRINT);

will output

1

which is the json_encoded value representing the number 1. The PHP documentation gives three more examples like this, returning values from the object, but since jsonSerialize() allows you to specify the actual data to be returned, it is important to realize that it can return anything. For example:

class JsonSerializeExample implements JsonSerializable {
    public function jsonSerialize() {
        return [
            'boolean' => true,
            'random_integer' => rand(),
            'int_from_object' => new IntegerValue(42),
            'another_object' => new stdClass,
        ];
    }
}
$obj = new JsonSerializeExample();
echo json_encode($obj, JSON_PRETTY_PRINT);

Will output

{
    "boolean": true,
    "random_integer": 1140562437,
    "int_from_object": 42,
    "another_object": {}
}

Of note is that random_integer is not a static value stored anywhere; it changes on each execution; and int_from_object demonstrates that json_encode() will recursively evaluate JsonSerializable instances.