JSON encoding and decoding in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does JSON encoding mean in PHP?

JSON encoding means converting PHP data like arrays or objects into a JSON string format that can be easily shared or stored.

beginner

Which PHP function is used to convert a PHP array into a JSON string?

The function json_encode() converts a PHP array or object into a JSON string.

beginner

What does JSON decoding mean in PHP?

JSON decoding means converting a JSON string back into a PHP variable like an array or object.

beginner

Which PHP function is used to convert a JSON string into a PHP variable?

The function json_decode() converts a JSON string into a PHP array or object.

intermediate

What does the second parameter of json_decode() control?

The second parameter controls if the JSON string is decoded into an associative array (true) or an object (false, default).

Which function converts a PHP array to a JSON string?

A json_encode()

B json_decode()

C json_parse()

D json_to_array()

json_encode() converts PHP data into a JSON string.

What does json_decode() return by default?

A A PHP associative array

B A PHP object

C A JSON string

D An error

By default, json_decode() returns a PHP object unless the second parameter is set to true.

How do you get an associative array from json_decode()?

A Set second parameter to true

B Set second parameter to false

C Use json_encode() instead

D Use json_parse()

Passing true as the second parameter to json_decode() returns an associative array.

What type of data can json_encode() convert?

A Only strings

B Only numbers

C Arrays and objects

D Only booleans

json_encode() can convert PHP arrays and objects into JSON strings.

What happens if json_decode() receives invalid JSON?

A Returns the original string

B Throws an exception

C Returns false

D Returns null

json_decode() returns null if the JSON string is invalid.

Explain how to convert a PHP array to JSON and back to a PHP array.

Think about the functions and parameters needed for encoding and decoding.

Describe the difference between decoding JSON to an object versus an associative array in PHP.

Focus on the second parameter of json_decode().

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