09 JSON Encoding

Goal

Convert PHP arrays and objects to and from JSON using the built-in json_encode/json_decode functions — no library required.

Prerequisites

• Arrays, Classes

Encoding

json_encode turns a PHP value into a JSON string. An associative array becomes a JSON object; an indexed (list-style) array becomes a JSON array:

<?php

$item = ["name" => "Widget", "price" => 9.99];
echo json_encode($item) . "\n";
// {"name":"Widget","price":9.99}

$list = ["Widget", "Gadget"];
echo json_encode($list) . "\n";
// ["Widget","Gadget"]

Encoding Objects

json_encode also accepts objects directly — it serializes public properties:

<?php

class Item {
    public function __construct(
        public readonly string $name,
        public readonly float $price,
    ) {}
}

$item = new Item("Widget", 9.99);
echo json_encode($item) . "\n";
// {"name":"Widget","price":9.99}

This works because readonly/public properties are visible to json_encode the same way they're visible to any other code — no toArray() step needed for the simple case. Reach for an explicit toArray() (from 07 Classes) when the JSON shape needs to differ from the object's internal shape.

Pretty-Printing

<?php

echo json_encode($item, JSON_PRETTY_PRINT) . "\n";

{
    "name": "Widget",
    "price": 9.99
}

Decoding

json_decode turns a JSON string back into a PHP value. By default it returns stdClass objects; pass true as the second argument to get associative arrays instead — almost always what you want:

<?php

$json = '{"name":"Widget","price":9.99}';

$asObject = json_decode($json);
echo $asObject->name . "\n"; // Widget

$asArray = json_decode($json, true);
echo $asArray["name"] . "\n"; // Widget

Handling Invalid JSON

json_decode returns null on invalid input rather than throwing — always check:

<?php

$data = json_decode("not valid json", true);

if ($data === null && json_last_error() !== JSON_ERROR_NONE) {
    echo "Invalid JSON: " . json_last_error_msg() . "\n";
}

Checkpoint

<?php

$items = [
    ["name" => "Widget", "price" => 9.99],
    ["name" => "Gadget", "price" => 19.99],
];

$json = json_encode($items, JSON_PRETTY_PRINT);
echo $json . "\n";

$decoded = json_decode($json, true);
echo count($decoded) . "\n"; // 2
echo $decoded[0]["name"] . "\n"; // Widget

Next

Continue to 10 Serving JSON.