Why Isset, empty, and is_null behavior in PHP? - Purpose & Use Cases

The Big Idea

Discover how simple checks can save you from confusing bugs and messy code!

The Scenario

Imagine you have a big list of user data, and you want to check if certain details like email or phone number are there before using them.

You try to do this by writing many if statements manually for each piece of data.

The Problem

Checking each value manually is slow and confusing because some values might be missing, empty, or set to null.

You might write many lines of code and still make mistakes, like treating an empty string as missing or missing data as null.

The Solution

Using isset, empty, and is_null helps you quickly and clearly check if a variable exists, is empty, or is null.

This saves time and avoids errors by giving you simple tools to handle different cases correctly.

Before vs After

✗ Before

$email = $user['email'] ?? null;
if ($email !== null && $email !== '') {
    // use email
}

✓ After

if (isset($user['email']) && !empty($user['email'])) {
    // use email
}

What It Enables

You can safely and easily check data presence and content, making your programs more reliable and easier to read.

Real Life Example

When building a signup form, you want to check if users entered their phone number before sending a confirmation SMS.

Using these functions helps you avoid errors if the phone number is missing or empty.

Key Takeaways

isset checks if a variable exists and is not null.

empty checks if a variable is missing, null, false, 0, or an empty string.

is_null checks if a variable is exactly null.

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