Null type and its meaning in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code with null?

Look at this PHP code. What will it print?

PHP

<?php
$value = null;
if (is_null($value)) {
    echo 'Yes';
} else {
    echo 'No';
}
?>

A No

B Yes

C null

D Error

💡 Hint

Check what is_null() does when the variable is null.

The function is_null() returns true if the variable is null. So it prints 'Yes'.

❓ Predict Output

intermediate

What is the output when comparing null with empty string?

What will this PHP code print?

PHP

<?php
var_dump(null == '');
var_dump(null === '');
?>

A bool(false) bool(false)

B bool(true) bool(true)

C bool(true) bool(false)

D bool(false) bool(true)

💡 Hint

Remember that == checks value equality with type juggling, === checks type and value.

Using ==, null and empty string are not considered equal (false). Using ===, types differ so false.

🔧 Debug

advanced

Why does this code cause a warning with null?

Find the reason for the warning in this PHP code:

PHP

<?php
$value = null;
echo strlen($value);
?>

A Warning because strlen() expects a string, null is not a string

B Warning because null cannot be echoed directly

C No warning, it prints 0

D Fatal error because null is undefined

💡 Hint

Check what type strlen() expects as input.

strlen() expects a string. Passing null causes a warning because null is not a string.

🧠 Conceptual

advanced

What does the null coalescing operator do in PHP?

Choose the best description of the null coalescing operator ?? in PHP.

A Assigns null to the left variable if the right is null

B Returns true if both values are null

C Returns the left value if it is not null, otherwise returns the right value

D Checks if both values are equal and not null

💡 Hint

Think about how to provide a default value when a variable might be null.

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

❓ Predict Output

expert

What is the output of this PHP code mixing null and arrays?

What will this PHP code print?

PHP

<?php
$array = ['a' => null, 'b' => 0, 'c' => false];
$count = 0;
foreach ($array as $key => $value) {
    if ($value == null) {
        $count++;
    }
}
echo $count;
?>

A 1

B 0

C 3

D 2

💡 Hint

Remember that == null is true for null and false, but not for 0 in PHP.

In PHP, == null is true for null and false, but not for 0. So keys 'a' and 'c' match, count is 2.

Practice

1. What does the null type represent in PHP?

easy

A. A variable with no value assigned

B. A variable holding the number zero

C. A variable holding an empty string

D. A variable holding a boolean false

Solution

1. Step 1: Understand the meaning of null in PHP

   Null means the variable does not have any value assigned to it.

2. Step 2: Differentiate null from other values

   Zero, empty string, and false are actual values, but null means no value.

3. Final Answer:

   A variable with no value assigned -> Option A

4. Quick Check:

   Null means no value = D [OK]

Hint: Null means no value, not zero or empty [OK]

Common Mistakes:

• Confusing null with zero
• Thinking null is an empty string
• Assuming null is false

2. Which of the following is the correct way to assign a null value to a variable in PHP?

easy

A. $var = 'null';

B. $var == null;

C. $var := null;

D. $var = null;

Solution

1. Step 1: Identify correct assignment syntax in PHP

   Assignment uses a single equals sign = in PHP.

2. Step 2: Check how to assign null

   Null is assigned as null without quotes, so $var = null; is correct.

3. Final Answer:

   $var = null; -> Option D

4. Quick Check:

   Assign null with = null; = C [OK]

Hint: Use single = and null without quotes [OK]

Common Mistakes:

• Using 'null' as a string
• Using == instead of = for assignment
• Using := which is invalid in PHP

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

$var = null;
if ($var === null) {
    echo 'Yes';
} else {
    echo 'No';
}

medium

A. Yes

B. No

C. null

D. Error

Solution

1. Step 1: Understand strict comparison with ===

   The operator === checks both value and type. Since $var is null, $var === null is true.

2. Step 2: Determine which branch runs

   Because condition is true, the code echoes 'Yes'.

3. Final Answer:

   Yes -> Option A

4. Quick Check:

   $var === null is true, so output is Yes [OK]

Hint: Use === to check exact null value [OK]

Common Mistakes:

• Using == which can be true for other falsy values
• Expecting output 'null' instead of 'Yes'
• Confusing echo output with variable value

4. Identify the error in this PHP code snippet:

$var = null
if ($var === null) {
    echo 'Null value';
}

medium

A. Incorrect comparison operator

B. Missing semicolon after assignment

C. Null cannot be assigned to variables

D. Echo statement syntax error

Solution

1. Step 1: Check syntax of assignment line

   The line $var = null is missing a semicolon at the end, which is required in PHP.

2. Step 2: Verify other parts of the code

   The comparison and echo statements are correct and valid.

3. Final Answer:

   Missing semicolon after assignment -> Option B

4. Quick Check:

   Every statement needs ; in PHP [OK]

Hint: Check for missing semicolons after statements [OK]

Common Mistakes:

• Forgetting semicolon after assignment
• Thinking null can't be assigned
• Misreading === as wrong operator

5. You want to check if a variable $data has no value assigned and then assign it a default string 'empty'. Which code correctly does this in PHP?

hard

A. if (is_null($data)) { $data = 'empty'; }

B. if ($data === null) { $data = 'empty'; }

C. All of the above

D. if ($data == null) { $data = 'empty'; }

Solution

1. Step 1: Understand ways to check for null

   In PHP, $data == null, $data === null, and is_null($data) all check if variable is null, with slight differences.

2. Step 2: Confirm assignment after check

   All three conditions correctly assign 'empty' if $data is null.

3. Step 3: Evaluate correctness of all options

   Since options B, C, and D work, the correct choice is 'All of the above'.

4. Final Answer:

   All of the above -> Option C

5. Quick Check:

   Multiple ways to check null = A [OK]

Hint: Use any null check method before assigning default [OK]

Common Mistakes:

• Thinking only === works for null check
• Ignoring is_null() function
• Confusing == with === always