JSON encoding and decoding in PHP

Introduction

JSON encoding and decoding lets you turn PHP data into a string format that computers can share easily, and then turn it back into PHP data.

When you want to send PHP data to a web browser or another program.

When you receive data from a web service and want to use it in PHP.

When you want to save PHP data in a file or database in a simple text format.

When you need to exchange data between different programming languages.

Syntax

PHP

$jsonString = json_encode($phpData);
$phpData = json_decode($jsonString, true);

json_encode() converts PHP data (like arrays or objects) into a JSON string.

json_decode() converts a JSON string back into PHP data. Use true as the second argument to get an array instead of an object.

Examples

This turns a PHP array into a JSON string.

PHP

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

This turns a JSON string back into a PHP array.

PHP

$json = '{"name":"Anna","age":25}';
$data = json_decode($json, true);
print_r($data);

Decoding without true returns an object. You can access properties with ->.

PHP

$object = json_decode($json);
echo $object->name;

Sample Program

This program shows how to encode a PHP array into JSON and then decode it back to a PHP array.

PHP

<?php
// Create PHP array
$user = ['username' => 'johndoe', 'email' => '[e-mail]', 'active' => true];

// Convert PHP array to JSON string
$jsonString = json_encode($user);
echo "JSON string:\n" . $jsonString . "\n\n";

// Convert JSON string back to PHP array
$decodedArray = json_decode($jsonString, true);
echo "Decoded PHP array:\n";
print_r($decodedArray);
?>

Important Notes

JSON only supports certain data types: strings, numbers, arrays, objects, booleans, and null.

When decoding, use true as the second argument to get an associative array instead of an object.

Always check if json_encode() or json_decode() returns false to catch errors.

Summary

Use json_encode() to turn PHP data into a JSON string.

Use json_decode() to turn a JSON string back into PHP data.

JSON is a simple way to share data between programs and languages.

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