(PHP 5 >= 5.4.0, PHP 7)
JsonSerializable::jsonSerialize — Specify data which should be serialized to JSON
Serializes the object to a value that can be serialized natively by json_encode().
This function has no parameters.
Returns data which can be serialized by json_encode(), which is a value of any type other than a resource.
Example #1 JsonSerializable::jsonSerialize() example returning an array
<?php
class ArrayValue implements JsonSerializable {
    public function __construct(array $array) {
        $this->array = $array;
    }
    public function jsonSerialize() {
        return $this->array;
    }
}
$array = [1, 2, 3];
echo json_encode(new ArrayValue($array), JSON_PRETTY_PRINT);
?>
The above example will output:
[
    1,
    2,
    3
]
Example #2 JsonSerializable::jsonSerialize() example returning an associative array
<?php
class ArrayValue implements JsonSerializable {
    public function __construct(array $array) {
        $this->array = $array;
    }
    public function jsonSerialize() {
        return $this->array;
    }
}
$array = ['foo' => 'bar', 'quux' => 'baz'];
echo json_encode(new ArrayValue($array), JSON_PRETTY_PRINT);
?>
The above example will output:
{
    "foo": "bar",
    "quux": "baz"
}
Example #3 JsonSerializable::jsonSerialize() example returning an integer
<?php
class IntegerValue implements JsonSerializable {
    public function __construct($number) {
        $this->number = (integer) $number;
    }
    public function jsonSerialize() {
        return $this->number;
    }
}
echo json_encode(new IntegerValue(1), JSON_PRETTY_PRINT);
?>
The above example will output:
1
Example #4 JsonSerializable::jsonSerialize() example returning a string
<?php
class StringValue implements JsonSerializable {
    public function __construct($string) {
        $this->string = (string) $string;
    }
    public function jsonSerialize() {
        return $this->string;
    }
}
echo json_encode(new StringValue('Hello!'), JSON_PRETTY_PRINT);
?>
The above example will output:
"Hello!"