Null coalescing operator in PHP - Mini Project: Build & Apply

Using the Null Coalescing Operator in PHP

📖 Scenario: Imagine you are building a simple PHP script that reads user input from an array. Sometimes the input might be missing, and you want to provide a default value instead of causing an error.

🎯 Goal: You will create a PHP script that uses the null coalescing operator ?? to safely access array values and provide default values when keys are missing.

📋 What You'll Learn

Create an associative array called userInput with specific keys and values

Create a variable called defaultName with a default string value

Use the null coalescing operator ?? to assign a variable name from userInput or defaultName

Print the value of name to display the result

💡 Why This Matters

🌍 Real World

Web developers often need to handle missing data safely when reading user input or configuration settings. The null coalescing operator helps avoid errors and provides default values easily.

💼 Career

Understanding how to use the null coalescing operator is important for writing clean, error-free PHP code in web development jobs.

1

Create the user input array

Create an associative array called userInput with these exact entries: 'name' => 'Alice', 'age' => 30.

PHP

 'Alice' and 'age' => 30
// Your code here

Hint

Use square brackets [] to create the array and separate key-value pairs with commas.

2

Set a default name

Create a variable called defaultName and set it to the string 'Guest'.

PHP

 'Alice', 'age' => 30];

// Create a variable defaultName with value 'Guest'
// Your code here

Hint

Use the assignment operator = to set the string value.

3

Use the null coalescing operator

Create a variable called name and assign it the value of $userInput['name'] ?? $defaultName using the null coalescing operator ??.

PHP

 'Alice', 'age' => 30];
$defaultName = 'Guest';

// Assign name using null coalescing operator to get $userInput['name'] or $defaultName
// Your code here

Hint

The null coalescing operator ?? returns the left value if it exists and is not null; otherwise, it returns the right value.

4

Print the result

Write a print statement to display the value of the variable name.

PHP

 'Alice', 'age' => 30];
$defaultName = 'Guest';
$name = $userInput['name'] ?? $defaultName;

// Print the value of name
// Your code here

Hint

Use print($name); to show the value on the screen.

Practice

1. What does the null coalescing operator ?? do in PHP?

easy

A. Returns the first operand if it is not null, otherwise returns the second operand.

B. Returns true if both operands are null.

C. Checks if a variable is set and throws an error if not.

D. Assigns null to a variable if it is not already set.

Solution

1. Step 1: Understand the operator's purpose

   The null coalescing operator ?? checks if the left value is null.

2. Step 2: Determine the returned value

   If the left value is not null, it returns that; otherwise, it returns the right value.

3. Final Answer:

   Returns the first operand if it is not null, otherwise returns the second operand. -> Option A

4. Quick Check:

   Null coalescing operator returns first non-null value [OK]

Hint: Remember: returns first non-null value [OK]

Common Mistakes:

• Thinking it returns true/false
• Confusing it with the ternary operator
• Assuming it throws errors if variable is unset

2. Which of the following is the correct syntax to use the null coalescing operator in PHP?

easy

A. $value = $a ??= $b;

B. $value = $a ?: $b;

C. $value = $a ? $b;

D. $value = $a ?? $b;

Solution

1. Step 1: Identify the null coalescing operator syntax

   The operator is written as ?? between two expressions.

2. Step 2: Check each option

   $value = $a ?? $b; uses $a ?? $b which is correct syntax. $value = $a ?: $b; uses the ternary shorthand, not null coalescing. $value = $a ? $b; is incomplete syntax. $value = $a ??= $b; uses ??= which is a different operator (null coalescing assignment).

3. Final Answer:

   $value = $a ?? $b; -> Option D

4. Quick Check:

   Correct null coalescing syntax uses ?? between values [OK]

Hint: Use ?? between two values for null coalescing [OK]

Common Mistakes:

• Confusing ?? with ?: operator
• Using incomplete or invalid syntax
• Mixing null coalescing with assignment operator

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

$a = null;
$b = 5;
$c = $a ?? $b;
echo $c;

medium

A. null

B. 0

C. 5

D. Error

Solution

1. Step 1: Evaluate the variables

   $a is null, $b is 5.

2. Step 2: Apply null coalescing operator

   $c = $a ?? $b means if $a is not null use $a, else use $b. Since $a is null, $c becomes 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Null coalescing returns 5 when left is null [OK]

Hint: If left is null, output right value [OK]

Common Mistakes:

• Assuming output is null
• Confusing with ternary operator behavior
• Expecting error due to null variable

4. Identify the error in the following PHP code using the null coalescing operator:

$value = ?? 10;

medium

A. Missing left operand before ?? operator.

B. Missing semicolon at the end.

C. Using wrong operator, should be ?: instead of ??.

D. No error, code is correct.

Solution

1. Step 1: Analyze the operator usage

   The null coalescing operator requires a left and right operand, like $a ?? $b.

2. Step 2: Identify the error in the code

   The code has ?? 10 without a left operand before ??, which is invalid syntax.

3. Final Answer:

   Missing left operand before ?? operator. -> Option A

4. Quick Check:

   Null coalescing needs both operands [OK]

Hint: Both sides needed for ?? operator [OK]

Common Mistakes:

• Leaving out left operand
• Confusing ?? with ternary operator
• Ignoring syntax errors

5. Given the array $data = ['name' => null, 'age' => 25];, which code snippet correctly assigns $result to the value of 'name' if it exists and is not null, otherwise to 'Unknown'?

hard

A. $result = $data['name'] ?? 'Unknown';

B. $result = isset($data['name']) ? $data['name'] : 'Unknown';

C. $result = $data['name'] ?: 'Unknown';

D. $result = $data['name'] ??= 'Unknown';

Solution

1. Step 1: Understand the difference between ?? and isset()

   The null coalescing operator returns the right value if the left is null or not set. However, if the key exists with a null value, $data['name'] ?? 'Unknown' returns 'Unknown'.

2. Step 2: Check each option's behavior

   $result = $data['name'] ?? 'Unknown'; returns 'Unknown' because 'name' is null. $result = isset($data['name']) ? $data['name'] : 'Unknown'; uses isset() which returns false if null, so it returns 'Unknown'. $result = $data['name'] ?: 'Unknown'; uses ?: which treats null as false, so returns 'Unknown'. $result = $data['name'] ??= 'Unknown'; uses ??= which assigns 'Unknown' to $data['name'] if null or unset, then returns it.

3. Step 3: Choose the best option for the requirement

   The question asks to assign $result to 'name' if it exists and is not null, else 'Unknown'. Since 'name' exists but is null, only $result = isset($data['name']) ? $data['name'] : 'Unknown'; correctly checks existence and returns 'Unknown'. $result = $data['name'] ?? 'Unknown'; and C treat null as missing, $result = $data['name'] ??= 'Unknown'; modifies the array which is not asked.

4. Final Answer:

   $result = isset($data['name']) ? $data['name'] : 'Unknown'; -> Option B

5. Quick Check:

   Use isset() to check existence and null separately [OK]

Hint: Use isset() to check key exists and not null [OK]

Common Mistakes:

• Assuming ?? checks key existence only
• Using ??= which changes original array
• Confusing ?: with ?? operator