JSON encoding and decoding in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using json_encode?

Consider the following PHP code snippet. What will it output?

PHP

<?php
$array = ['name' => 'Alice', 'age' => 30, 'active' => true];
echo json_encode($array);
?>

A {"name":"Alice","age":30,"active":true}

B {"name":"Alice","age":"30","active":"true"}

C {"name":Alice,"age":30,"active":true}

D Array

💡 Hint

Remember json_encode converts PHP arrays to JSON strings with proper quotes and types.

json_encode converts the PHP associative array into a JSON string with keys and values properly quoted and boolean true as true without quotes.

❓ Predict Output

intermediate

What does json_decode return with second parameter true?

What will be the output of this PHP code?

PHP

<?php
$json = '{"fruit":"apple","count":5}';
$result = json_decode($json, true);
var_dump($result);
?>

A array(2) { ["fruit"]=> string(5) "apple" ["count"]=> int(5) }

B object(stdClass)#1 (2) { ["fruit"]=> string(5) "apple" ["count"]=> int(5) }

C string(23) "{"fruit":"apple","count":5}"

D NULL

💡 Hint

The second parameter true makes json_decode return an associative array.

json_decode with true returns an associative array, so var_dump shows array with keys and values.

❓ Predict Output

advanced

What error does this PHP code raise when decoding invalid JSON?

What error or output will this PHP code produce?

PHP

<?php
$invalidJson = '{"name":"Bob", age: 25}';
$result = json_decode($invalidJson);
if (json_last_error() !== JSON_ERROR_NONE) {
    echo json_last_error_msg();
} else {
    var_dump($result);
}
?>

A NULL

B Control character error, possibly malformed UTF-8 characters

C Syntax error, malformed JSON

D Syntax error

💡 Hint

Look at the JSON string: keys must be quoted strings.

The JSON string is invalid because the key 'age' is not quoted, causing a syntax error in JSON parsing.

🧠 Conceptual

advanced

Which option correctly encodes a PHP object to JSON with private properties?

Given a PHP class with private properties, which code snippet will correctly encode its public data to JSON?

PHP

<?php
class User {
    private string $name;
    private int $age;
    public function __construct(string $name, int $age) {
        $this->name = $name;
        $this->age = $age;
    }
    public function toArray(): array {
        return ['name' => $this->name, 'age' => $this->age];
    }
}
$user = new User('Eve', 28);
// Which line below correctly encodes the user data?

A echo json_encode($user);

B echo json_encode($user->toArray());

C echo json_encode(get_object_vars($user));

D echo json_encode((array)$user);

💡 Hint

Private properties are not accessible directly; use a method to expose data.

json_encode only encodes public properties by default. Using toArray() exposes data as an array for encoding.

🚀 Application

expert

How many items are in the array after decoding this JSON string?

Given this PHP code, how many items does the resulting array contain?

PHP

<?php
$json = '[{"id":1,"tags":["php","json"]},{"id":2,"tags":[]},{"id":3}]';
$array = json_decode($json, true);
echo count($array);
?>

A 1

B 2

C 3

D 0

💡 Hint

Count the top-level elements in the JSON array.

The JSON string is an array with 3 objects, so count($array) returns 3.

Practice

1. What does the PHP function json_encode() do?

easy

A. Converts PHP data into a JSON formatted string

B. Converts JSON string into PHP data

C. Validates JSON syntax

D. Reads JSON data from a file

Solution

1. Step 1: Understand the purpose of json_encode()

   This function takes PHP variables like arrays or objects and converts them into a JSON string format.

2. Step 2: Differentiate from json_decode()

   json_decode() does the opposite by converting JSON strings back to PHP data.

3. Final Answer:

   Converts PHP data into a JSON formatted string -> Option A

4. Quick Check:

   json_encode() = PHP to JSON string [OK]

Hint: json_encode() makes PHP data into JSON text [OK]

Common Mistakes:

• Confusing json_encode() with json_decode()
• Thinking json_encode() reads JSON from files
• Assuming json_encode() validates JSON syntax

2. Which of the following is the correct syntax to decode a JSON string into a PHP array?

easy

A. $data = json_encode($jsonString, true);

B. $data = json_decode($jsonString, true);

C. $data = json_decode($jsonString);

D. $data = json_encode($jsonString);

Solution

1. Step 1: Identify the function to decode JSON

   The function to convert JSON string to PHP data is json_decode().

2. Step 2: Use the second parameter to get an array

   Passing true as the second argument returns an associative array instead of an object.

3. Final Answer:

   $data = json_decode($jsonString, true); -> Option B

4. Quick Check:

   json_decode() with true returns array [OK]

Hint: Use json_decode($json, true) for PHP arrays [OK]

Common Mistakes:

• Using json_encode() instead of json_decode()
• Omitting the second parameter to get an array
• Passing true to json_encode() which is invalid

3. What will be the output of the following PHP code?

$arr = ['name' => 'Anna', 'age' => 25];
$json = json_encode($arr);
echo $json;

medium

A. {"name":"Anna","age":25}

B. Array

C. name: Anna, age: 25

D. Error: Cannot encode array

Solution

1. Step 1: Understand json_encode output

   The function converts the PHP associative array into a JSON string with keys and values in double quotes.

2. Step 2: Check the exact JSON string format

   The output is a JSON object string: {"name":"Anna","age":25} with escaped quotes when printed.

3. Final Answer:

   {"name":"Anna","age":25} -> Option A

4. Quick Check:

   json_encode() outputs JSON string [OK]

Hint: json_encode() outputs JSON string with quotes [OK]

Common Mistakes:

• Expecting PHP array print instead of JSON string
• Missing quotes around keys and values
• Confusing output with plain text or error

4. Identify the error in this PHP code snippet:

$json = '{"fruit":"apple","count":3}';
$data = json_decode($json, true);
echo $data->fruit;

medium

A. json_decode cannot parse this JSON

B. Incorrect JSON string format

C. Missing second parameter in json_decode

D. Using object syntax on an array after json_decode

Solution

1. Step 1: Check json_decode parameters

   The second parameter true makes json_decode return an associative array.

2. Step 2: Accessing array with object syntax

   Using $data->fruit tries to access an object property, but $data is an array, so this causes an error.

3. Final Answer:

   Using object syntax on an array after json_decode -> Option D

4. Quick Check:

   json_decode(true) returns array, use $data['fruit'] [OK]

Hint: Use $data['key'] if json_decode(true) used [OK]

Common Mistakes:

• Accessing array as object after json_decode(true)
• Assuming JSON string is invalid
• Forgetting to pass true for array output

5. You have a PHP array:

$data = ["id" => 101, "tags" => ["php", "json", "coding"]];

Which code correctly encodes this array to JSON and then decodes it back to a PHP object?

hard

A. $json = json_encode($data);
$obj = json_decode($json, true);

B. $json = json_decode($data);
$obj = json_encode($json);

C. $json = json_encode($data);
$obj = json_decode($json);

D. $json = json_encode($data, true);
$obj = json_decode($json, true);

Solution

1. Step 1: Encode PHP array to JSON string

   Use json_encode($data) to convert the PHP array into a JSON string.

2. Step 2: Decode JSON string back to PHP object

   Calling json_decode($json) without the second parameter returns a PHP object.

3. Final Answer:

   $json = json_encode($data); $obj = json_decode($json); -> Option C

4. Quick Check:

   Encode then decode without true returns object [OK]

Hint: json_decode() without true returns object [OK]

Common Mistakes:

• Using json_decode before json_encode
• Passing true to json_encode (invalid)
• Decoding with true returns array, not object