Working with JSON

JSON is the universal language of web APIs. PHP gives you exactly two functions for it — json_encode to send data out and json_decode to read it back in. Master those two, plus error handling, and you can talk to any API on the internet.

Learn Working with JSON in our free PHP course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick recall.

Part of the free Php course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1️⃣ Encoding to JSON

json_encode($value) serialises a PHP value into a JSON string. An associative array becomes a JSON object {' '} , a list array becomes a JSON array [ ] , and scalars map directly. Pass the JSON_PRETTY_PRINT flag for nicely indented output when a human needs to read it.

2️⃣ Decoding from JSON

json_decode($string) goes the other way. With one argument you get a stdClass object (access with -> ); pass true as the second argument to get an associative array instead (access with [] ). Most PHP code uses the array form — it's easier to loop and pass around.

3️⃣ Handling Invalid JSON

Here's a trap: when JSON is malformed, json_decode returns null silently — and null is also a valid JSON value, so a bare null-check is ambiguous. Either inspect json_last_error() , or — much cleaner on modern PHP — pass JSON_THROW_ON_ERROR so bad input throws a JsonException you can catch.

4️⃣ The Round-Trip

Almost every API endpoint is a round-trip: take a PHP object, json_encode it for the response, then on the next request json_decode the body back into PHP. Note that json_encode serialises only an object's public properties — private and protected ones are skipped (you'd implement JsonSerializable to customise that).

Now you try — fill in each ___ using the 👉 hint, then run it and check against the Output panel.

These lines round-trip a value through JSON — but they're scrambled. Put them in the order that prints Ada .

Build the array ( B ), encode it to a JSON string ( D ), decode that string back into an array ( C ), then read the value out ( A ). Each step depends on the variable produced by the one before it.

{' '} — string keys produce a JSON object, and the PHP true becomes JSON true .

object — without the true flag, json_decode returns a stdClass object. You'd read $x->n , not $x['n'] .

NULL — invalid JSON makes json_decode return null silently. That's why you check json_last_error() or use JSON_THROW_ON_ERROR .

📋 Quick Reference — JSON

No code is filled in this time — just a brief and an outline. Write it yourself, run it on onecompiler.com/php or your own machine, then check your result against the expected output in the comments.

Practice quiz

What does json_decode return by default (with no second argument)?

  • An associative array
  • A string
  • A stdClass object
  • null

Answer: A stdClass object. By default json_decode returns a stdClass object, so you read properties with the arrow: $data->name. Pass true for an array.

What does json_decode($json, true) return?

  • An associative array
  • A stdClass object
  • A JSON string
  • false

Answer: An associative array. Passing true as the second argument gives an associative array, read with brackets: $data['name'].

What does echo json_encode(['a' => 1, 'b' => true]); print?

  • {"a":1,"b":1}

String keys produce a JSON object, and the PHP boolean true becomes JSON true (not 1).

What does json_decode return when given invalid JSON?

  • false
  • null (silently)
  • An empty array
  • It throws by default

Answer: null (silently). Invalid JSON makes json_decode return null silently — which is why you check json_last_error() or use JSON_THROW_ON_ERROR.

Which flag makes json_decode throw a JsonException on bad input instead of returning null?

  • JSON_THROW_ON_ERROR
  • JSON_PRETTY_PRINT
  • JSON_UNESCAPED_UNICODE
  • JSON_ERROR_NONE

Answer: JSON_THROW_ON_ERROR. JSON_THROW_ON_ERROR makes malformed input throw a JsonException you can catch, rather than silently returning null.

Which object properties does json_encode serialise by default?

  • All properties
  • Only private properties
  • Only public properties
  • Only properties listed in a __toString

Answer: Only public properties. json_encode serialises only public properties; private and protected ones are skipped. Implement JsonSerializable to customise.

Which flag makes json_encode output indented, human-readable JSON?

  • JSON_INDENT
  • JSON_PRETTY_PRINT
  • JSON_FORMAT
  • JSON_HUMAN

Answer: JSON_PRETTY_PRINT. JSON_PRETTY_PRINT formats the output with indentation for readability.

How do you combine multiple json_encode flags?

  • With commas
  • With +
  • You can only pass one flag
  • With the bitwise OR operator |

Answer: With the bitwise OR operator |. Flags combine with bitwise OR, e.g. json_encode($data, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE).

Why is a bare null check after json_decode ambiguous?

  • null is never returned
  • The literal JSON value null also decodes to null
  • json_decode returns false on error
  • null means success

Answer: The literal JSON value null also decodes to null. json_decode returns null both for the literal JSON null and for invalid JSON, so use json_last_error() to tell them apart.

Which interface lets you customise exactly what an object encodes to?

  • Serializable
  • Stringable
  • JsonSerializable
  • ArrayAccess

Answer: JsonSerializable. Implement JsonSerializable and define jsonSerialize() to return the exact array or value you want encoded.