Null coalescing operator in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign $value to $result if $value is set, otherwise assign 'default'.

PHP

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

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

A||

B??

C&&

D?:

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' instead of '??' which checks for truthiness, not null.

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 code to get the value of $_GET['name'] or use 'Guest' if it is not set.

PHP

$name = $_GET['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 checks for truthiness and can cause unexpected results.

Using '?:' which requires the left operand to be evaluated differently.

The null coalescing 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 code to safely assign $username from $_POST['user'] or 'anonymous' if not set.

PHP

$username = $_POST['user'] [1] 'anonymous';

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

A||

B&&

C?:

D??

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' which can cause warnings if the key does not exist.

Using '?:' which requires the left operand to be set and truthy.

Using '??' avoids undefined index warnings by checking if the key exists and is not null.

4fill in blank

hard

Fill both blanks to create an array $settings with keys 'theme' and 'language' using null coalescing operator to provide defaults.

PHP

$settings = [
    'theme' => $userSettings['theme'] [1] 'light',
    'language' => $userSettings['language'] [2] 'en'
];

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

A??

B||

C?:

D&&

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' which checks truthiness and can cause unexpected results.

Using '?:' which behaves differently and may cause warnings.

The '??' operator safely returns the value if set and not null, otherwise the default value.

5fill in blank

hard

Fill all three blanks to assign $finalName using nested null coalescing operators to check $input['first'], $input['nickname'], or default to 'User'.

PHP

$finalName = $input['first'] [1] $input['nickname'] [2] [3];

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

A??

C'User'

D||

Attempts:

3 left

💡 Hint

Common Mistakes

Using '||' which checks truthiness and can cause unexpected results.

Not nesting the operators correctly.

Nested '??' operators check each value in order, returning the first that exists and is not null, else the default string.

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