Null coalescing operator in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of null coalescing with undefined variable

What is the output of this PHP code snippet?

PHP

<?php
$a = null;
echo $a ?? 'default';
?>

A default

B null

C Notice: Undefined variable

D 0

💡 Hint

The null coalescing operator returns the right side if the left is null or undefined.

Since $a is explicitly set to null, the null coalescing operator returns the right side 'default'.

❓ Predict Output

intermediate

Null coalescing with multiple variables

What will this PHP code output?

PHP

<?php
$x = null;
$y = 0;
$z = 'hello';
echo $x ?? $y ?? $z;
?>

A hello

B 0

C None

D Notice: Undefined variable

💡 Hint

The operator returns the first value that is not null.

Here, $x is null, so it checks $y which is 0 (not null), so it outputs 0.

❓ Predict Output

advanced

Null coalescing with array keys

What is the output of this PHP code?

PHP

<?php
$arr = ['a' => null, 'b' => 5];
echo $arr['a'] ?? $arr['b'] ?? 10;
?>

A 10

B null

C 5

D Notice: Undefined index

💡 Hint

Null coalescing operator checks if the key exists and is not null.

The key 'a' exists but is null, so it moves to 'b' which is 5 and outputs 5.

❓ Predict Output

advanced

Null coalescing with undefined array key

What will this PHP code output?

PHP

<?php
$arr = ['x' => 1];
echo $arr['y'] ?? 'missing';
?>

A missing

B Notice: Undefined index

C 1

D null

💡 Hint

Null coalescing operator avoids notice for undefined keys.

Since 'y' key does not exist, it returns the right side 'missing' without error.

❓ Predict Output

expert

Null coalescing with function return values

What is the output of this PHP code?

PHP

<?php
function getValue() {
    return false;
}

$result = getValue() ?? 'default';
echo $result;
?>

A Notice: Undefined variable

B default

C 0

D false

💡 Hint

Null coalescing checks for null, not false or zero.

The function returns false which is not null, so the operator returns false and echoes it as empty 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