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

Null coalescing in conditions

📖 Scenario: You are building a simple PHP script to check user input from a form. Sometimes the input might be missing or empty. You want to use the null coalescing operator to provide a default value when the input is not set.

🎯 Goal: Create a PHP script that uses the null coalescing operator ?? in a condition to check if a user input variable is set. If it is not set, use a default value. Then print a message based on the value.

📋 What You'll Learn

Create a variable $input with a specific value

Create a variable $default with a default string

Use the null coalescing operator ?? in an if condition to check $input or use $default

Print the message showing which value was used

💡 Why This Matters

🌍 Real World

Web forms often have optional fields. Using null coalescing helps handle missing inputs gracefully.

💼 Career

PHP developers use null coalescing to write cleaner, safer code when dealing with user input or data that might be missing.

1

Create the user input variable

Create a variable called $input and set it to null.

PHP

<?php

// Create a variable called $input and set it to null
// Your code here

Hint

Use $input = null; to create the variable.

2

Create the default value variable

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

PHP

<?php

$input = null;
// Create a variable called $default and set it to 'Guest'
// Your code here

Hint

Use $default = 'Guest'; to create the variable.

3

Use null coalescing in an if condition

Write an if statement that uses the null coalescing operator ?? to check if $input is set; if not, use $default. Inside the if, check if the result equals 'Guest'.

PHP

<?php

$input = null;
$default = 'Guest';

// Use if with null coalescing operator to check $input or $default
// Check if the result equals 'Guest'
// Your code here

Hint

Use if (($input ?? $default) === 'Guest') to check the value.

4

Print the result message

Inside the if block, print "Hello, Guest!". Otherwise, print "Hello, " followed by the value of $input.

PHP

<?php

$input = null;
$default = 'Guest';

if (($input ?? $default) === 'Guest') {
    // Print Hello, Guest!
} else {
    // Print Hello, $input
}

Hint

Use print("Hello, Guest!"); inside the if and print("Hello, $input"); inside the else.

Practice

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

easy

A. Always returns the second operand regardless of the first.

B. Returns the first operand if it exists and is not null; otherwise returns the second operand.

C. Checks if both operands are null and returns true if so.

D. Throws an error if the first operand is null.

Solution

1. Step 1: Understand the operator's purpose

   The null coalescing operator ?? checks if the left value exists and is not null.

2. Step 2: Determine the return value

   If the left value is null or missing, it returns the right value as a fallback.

3. Final Answer:

   Returns the first operand if it exists and is not null; otherwise returns the second operand. -> Option B

4. Quick Check:

   Null coalescing = first non-null value [OK]

Hint: Think: "Use first value if valid, else fallback" [OK]

Common Mistakes:

• Confusing it with logical OR operator
• Assuming it throws errors on null
• Believing it always returns second operand

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

easy

A. if ($value || 'default') { ... }

B. if ($value ?: 'default') { ... }

C. if ($value ?? 'default') { ... }

D. if ($value ??= 'default') { ... }

Solution

1. Step 1: Identify null coalescing syntax

   The null coalescing operator is ?? and used like $value ?? 'default'.

2. Step 2: Check each option

   if ($value ?? 'default') { ... } uses ?? correctly inside the if condition. if ($value ?: 'default') { ... } uses the ternary shortcut, not null coalescing. if ($value || 'default') { ... } uses logical OR, which is different. if ($value ??= 'default') { ... } uses ??= which is an assignment operator, not a condition.

3. Final Answer:

   if ($value ?? 'default') { ... } -> Option C

4. Quick Check:

   Correct null coalescing syntax = if ($value ?? 'default') { ... } [OK]

Hint: Look for ?? operator inside the if condition [OK]

Common Mistakes:

• Mixing null coalescing with ternary or OR operators
• Using assignment operator ??= inside if
• Missing parentheses or wrong operator

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

$name = null;
$display = $name ?? 'Guest';
echo $display;

medium

A. Guest

B. Empty string

C. Error

D. null

Solution

1. Step 1: Evaluate the variable $name

   The variable $name is set to null.

2. Step 2: Apply null coalescing operator

   The expression $name ?? 'Guest' returns 'Guest' because $name is null.

3. Final Answer:

   Guest -> Option A

4. Quick Check:

   null ?? 'Guest' = 'Guest' [OK]

Hint: Null on left means right value is chosen [OK]

Common Mistakes:

• Thinking null prints as 'null' string
• Confusing null coalescing with OR operator
• Expecting error when variable is null

4. Find the error in this PHP code snippet using null coalescing:

if ($user ?? 'guest') {
    echo 'Welcome!';
}

medium

A. The condition always evaluates to true, so it may not work as intended.

B. The variable $user must be checked with isset() before using ??.

C. The null coalescing operator cannot be used inside if conditions.

D. The operator ?? should be replaced with logical OR ||.

Solution

1. Step 1: Understand the condition logic

   The expression $user ?? 'guest' returns $user if set and not null, else 'guest'.

2. Step 2: Analyze boolean evaluation

   Since 'guest' is a non-empty string, the condition is always true, even if $user is null or false.

3. Final Answer:

   The condition always evaluates to true, so it may not work as intended. -> Option A

4. Quick Check:

   Non-null fallback makes condition always true [OK]

Hint: Check if fallback value makes condition always true [OK]

Common Mistakes:

• Assuming null coalescing checks boolean truth
• Thinking isset() is required with ?? operator
• Replacing ?? with || without reason

5. You want to display a username if it exists and is not null, otherwise show 'Anonymous'. Which code correctly uses null coalescing in a condition to echo the username or fallback?

hard

A. echo $username || 'Anonymous';

B. echo isset($username) ? $username : 'Anonymous';

C. echo $username ?: 'Anonymous';

D. echo $username ?? 'Anonymous';

Solution

1. Step 1: Understand the requirement

   We want to print $username if it exists and is not null; otherwise print 'Anonymous'.

2. Step 2: Compare options

   echo isset($username) ? $username : 'Anonymous'; uses ternary with isset(), which works but is longer. echo $username ?? 'Anonymous'; uses null coalescing ?? which directly checks for null and provides fallback. echo $username ?: 'Anonymous'; uses the ternary shortcut ?: which treats empty strings or 0 as false, not just null. echo $username || 'Anonymous'; uses logical OR which is incorrect syntax for echo.

3. Final Answer:

   echo $username ?? 'Anonymous'; -> Option D

4. Quick Check:

   Null coalescing for fallback = echo $username ?? 'Anonymous'; [OK]

Hint: Use ?? to fallback only on null, not falsy values [OK]

Common Mistakes:

• Using ?: which treats empty strings as false
• Using logical OR which is invalid here
• Overcomplicating with isset() ternary