Isset, empty, and is_null behavior in PHP - Mini Project: Build & Apply

Isset, empty, and is_null behavior

📖 Scenario: Imagine you are managing user data in a PHP application. You need to check if certain user details exist, are empty, or are null before processing them.

🎯 Goal: Learn how to use isset, empty, and is_null functions in PHP to check variables correctly.

📋 What You'll Learn

Create an associative array called $user with specific keys and values

Create a variable $key to check in the array

Use isset, empty, and is_null functions to check the value of $user[$key]

Print the results of these checks clearly

💡 Why This Matters

🌍 Real World

Checking if user input or data exists and is valid before using it is very common in web applications.

💼 Career

Understanding how to check variables properly helps avoid bugs and errors in PHP programming jobs.

1

Create the user data array

Create an associative array called $user with these exact entries: 'name' => 'Alice', 'age' => 0, 'email' => '', 'phone' => null.

PHP

# Create the user array with the specified keys and values
# Your code here

Hint

Use square brackets to create the array and exact keys and values as given.

2

Set the key to check

Create a variable called $key and set it to the string 'age'.

PHP

 'Alice', 'age' => 0, 'email' => '', 'phone' => null];
# Set the key to check
# Your code here

Hint

Assign the string 'age' to the variable $key.

3

Check the variable using isset, empty, and is_null

Use isset($user[$key]), empty($user[$key]), and is_null($user[$key]) to check the value of $user[$key]. Store the results in variables called $isSet, $isEmpty, and $isNull respectively.

PHP

 'Alice', 'age' => 0, 'email' => '', 'phone' => null];
$key = 'age';
# Use isset, empty, and is_null to check $user[$key]
# Your code here

Hint

Use the exact variable names and functions as shown.

4

Print the results

Print the values of $isSet, $isEmpty, and $isNull using var_export to show true or false clearly.

PHP

 'Alice', 'age' => 0, 'email' => '', 'phone' => null];
$key = 'age';
$isSet = isset($user[$key]);
$isEmpty = empty($user[$key]);
$isNull = is_null($user[$key]);
# Print the results using var_export
# Your code here

Hint

Use var_export($variable, true) inside print to show boolean values as 'true' or 'false'.

Practice

1. Which of the following statements about isset in PHP is true?

easy

A. isset returns true even if the variable is null.

B. isset returns false if the variable is not set or is null.

C. isset checks if a variable is empty or zero.

D. isset returns true only if the variable is a string.

Solution

1. Step 1: Understand isset behavior

   isset returns false if the variable does not exist or is set to null.

2. Step 2: Analyze options

   isset returns false if the variable is not set or is null. correctly states this behavior: isset returns false if the variable is not set or is null. Options A, C, and D are incorrect because isset returns false for null, does not check emptiness or zero, and is not limited to strings.

3. Final Answer:

   isset returns false if the variable is not set or is null. -> Option B

4. Quick Check:

   isset false if null or unset [OK]

Hint: Remember: isset false if variable missing or null [OK]

Common Mistakes:

• Thinking isset returns true for null variables
• Confusing isset with empty
• Assuming isset checks variable type

2. Which of the following is the correct syntax to check if a variable $var is empty in PHP?

easy

A. if (empty($var)) { ... }

B. if (empty->$var) { ... }

C. if (is_empty($var)) { ... }

D. if (empty $var) { ... }

Solution

1. Step 1: Recall empty syntax

   The empty function requires parentheses around the variable: empty($var).

2. Step 2: Check each option

   if (empty($var)) { ... } uses correct syntax. if (empty $var) { ... } misses parentheses, C uses a non-existent function, and B uses invalid syntax.

3. Final Answer:

   if (empty($var)) { ... } -> Option A

4. Quick Check:

   empty requires parentheses [OK]

Hint: Use parentheses with empty: empty($var) [OK]

Common Mistakes:

• Omitting parentheses with empty
• Using is_empty instead of empty
• Using arrow operator incorrectly

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

$a = 0;
var_dump(isset($a));
var_dump(empty($a));
var_dump(is_null($a));

medium

A. bool(true)bool(false)bool(false)

B. bool(false)bool(true)bool(true)

C. bool(false)bool(false)bool(true)

D. bool(true)bool(true)bool(false)

Solution

1. Step 1: Evaluate isset($a) when $a = 0

   $a is set and not null, so isset($a) returns true.

2. Step 2: Evaluate empty($a) when $a = 0

   Zero is considered empty in PHP, so empty($a) returns true.

3. Step 3: Evaluate is_null($a) when $a = 0

   $a is integer zero, not null, so is_null($a) returns false.

4. Final Answer:

   bool(true)
   bool(true)
   bool(false) -> Option D

5. Quick Check:

   isset true, empty true for 0, is_null false [OK]

Hint: 0 is set and empty, but not null [OK]

Common Mistakes:

• Thinking isset returns false for 0
• Confusing empty with is_null
• Assuming 0 is null

4. Consider the following PHP code snippet:

$var = null;
if (empty($var)) {
    echo 'Empty';
} else {
    echo 'Not Empty';
}

Why does this code print "Empty"?

medium

A. Because empty returns false for null values.

B. Because empty only checks if variable is set.

C. Because empty returns true for null values.

D. Because empty throws an error for null values.

Solution

1. Step 1: Understand empty behavior with null

   empty returns true if the variable is not set or its value is considered empty, including null.

2. Step 2: Analyze the code output

   Since $var is null, empty($var) returns true, so "Empty" is printed.

3. Final Answer:

   Because empty returns true for null values. -> Option C

4. Quick Check:

   empty true for null [OK]

Hint: empty true for null or unset variables [OK]

Common Mistakes:

• Thinking empty only checks if variable is set
• Assuming empty returns false for null
• Expecting error on null with empty

5. You want to check if a variable $x exists and is not null, but also want to treat the value 0 as valid (not empty). Which combination of PHP functions correctly achieves this?

hard

A. if (isset($x) && $x !== null) { ... }

B. if (isset($x) && !empty($x)) { ... }

C. if (!is_null($x) && !empty($x)) { ... }

D. if (!empty($x) || isset($x)) { ... }

Solution

1. Step 1: Understand the requirement

   We want to check if $x exists and is not null, but 0 should be valid (not treated as empty).

2. Step 2: Analyze each option

   if (isset($x) && !empty($x)) { ... } fails because empty(0) is true, so 0 would be excluded. if (isset($x) && $x !== null) { ... } checks if $x is set and not null, allowing 0. if (!is_null($x) && !empty($x)) { ... } excludes 0 because of empty($x). if (!empty($x) || isset($x)) { ... } uses OR, which can be true even if $x is empty or null.

3. Final Answer:

   if (isset($x) && $x !== null) { ... } -> Option A

4. Quick Check:

   Use isset and strict null check to allow 0 [OK]

Hint: Use isset and !== null to allow zero values [OK]

Common Mistakes:

• Using empty excludes zero values
• Using OR instead of AND
• Confusing is_null with isset