Null type and its meaning in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the null type represent in PHP?

The null type represents a variable with no value or an empty value. It means the variable is explicitly set to have no data.

beginner

How do you assign a null value to a variable in PHP?

You assign null by writing: $var = null;. This means $var holds no value.

intermediate

What is the difference between an uninitialized variable and a variable set to null in PHP?

An uninitialized variable has never been given a value, while a variable set to null has been explicitly assigned no value.

beginner

What does the function is_null() do in PHP?

The is_null() function checks if a variable is null. It returns true if the variable is null, otherwise false.

beginner

Can a variable of any type be set to null in PHP?

Yes, in PHP any variable can be set to null, which means it no longer holds its previous value.

What does null mean in PHP?

A Variable holds false

B Variable holds zero

C Variable holds an empty string

D Variable has no value

null means the variable has no value at all, not zero, empty string, or false.

How do you check if a variable is null in PHP?

A Using <code>isset()</code>

B Using <code>empty()</code>

C Using <code>is_null()</code>

D Using <code>is_string()</code>

is_null() returns true if the variable is null.

What happens if you assign null to a variable that had a value before?

A It keeps the old value

B It loses its previous value

C It becomes zero

D It becomes an empty array

Assigning null removes the previous value from the variable.

Which of these is NOT true about null in PHP?

A <code>null</code> is the same as false

B <code>null</code> means no value

C Variables can be set to <code>null</code>

D <code>is_null()</code> checks for <code>null</code>

null is different from false; false is a boolean, null means no value.

What is the result of isset($var) if $var = null;?

A false

B true

C null

D error

isset() returns false if the variable is null.

Explain what the null type means in PHP and how it is used.

Think about how you tell PHP a variable has no data.

Describe how to check if a variable is null and what happens when you assign null to a variable.

Consider the functions and effects on the variable.

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