Null coalescing in conditions in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign $value to $input if it exists, otherwise assign 'default'.

PHP

$value = $input [1] 'default';

Drag options to blanks, or click blank then click option'

A&&

B?:

C||

D??

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' which checks for truthiness but can cause unexpected results.

Using '?:' which is the ternary shorthand but behaves differently.

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

2fill in blank

medium

Complete the condition to check if $data['name'] exists, otherwise use 'Guest'.

PHP

echo $data['name'] [1] 'Guest';

Drag options to blanks, or click blank then click option'

A?:

B??

C||

D&&

Attempts:

3 left

💡 Hint

Common Mistakes

Using '?:' which requires the left side to be truthy, not just set.

Using '||' which can cause type coercion issues.

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

3fill in blank

hard

Fix the error in the condition to assign $result to $input if set, else 'none'.

PHP

$result = [1] ?? 'none';

Drag options to blanks, or click blank then click option'

Aisset($input)

B$input ??

C$input

Dempty($input)

Attempts:

3 left

💡 Hint

Common Mistakes

Using B, which places ?? twice causing a syntax error.

Using functions like isset($input), which check existence but do not return the value.

Use the variable $input directly on the left side of the null coalescing operator ??.

4fill in blank

hard

Complete the code to assign $output to $array['key'] if set, else 'missing'.

PHP

$output = $array['key'][1] 'missing';

Drag options to blanks, or click blank then click option'

A[

B]

C??

D?:

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of brackets for array access.

Using the ternary operator instead of null coalescing.

Use square brackets to access the array key and the null coalescing operator to provide a default value.

5fill in blank

hard

Complete the code to create a condition that echoes $user['name'] if set, else 'Anonymous', and assigns it to $displayName.

PHP

$displayName = $user['name'] 'Anonymous';
echo [1];

Drag options to blanks, or click blank then click option'

A[

B??

C$displayName

D]

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to close the bracket for array access.

Echoing the array access expression instead of the variable.

Access the array key with brackets, use the null coalescing operator for default, and echo the assigned variable.

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