Nullable types in functions in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with nullable int parameter

What is the output of this PHP code when calling describeAge(null);?

PHP

<?php
function describeAge(?int $age): string {
    if ($age === null) {
        return "Age is unknown.";
    }
    return "Age is $age.";
}
echo describeAge(null);

A Age is unknown.

B Age is 0.

C Age is null.

D TypeError

💡 Hint

Remember that the question mark before int means the parameter can be null.

The function accepts an int or null. When null is passed, it returns 'Age is unknown.' because of the explicit check.

❓ Predict Output

intermediate

Return type with nullable string

What will this PHP code output when calling getName(null);?

PHP

<?php
function getName(?string $name): ?string {
    if ($name === null) {
        return null;
    }
    return strtoupper($name);
}
var_dump(getName(null));

A TypeError

B "NULL"

C string(0) ""

D NULL

💡 Hint

The function returns null explicitly if input is null.

The function returns null when input is null, so var_dump outputs NULL without quotes.

❓ Predict Output

advanced

Nullable type with default value

What is the output of this PHP code when calling greet();?

PHP

<?php
function greet(?string $name = null): string {
    return $name ? "Hello, $name!" : "Hello, guest!";
}
echo greet();

A Hello, guest!

B Hello, !

C Hello, null!

D TypeError

💡 Hint

The default value is null and the function checks if $name is truthy.

Since no argument is passed, $name is null, which is falsy, so it returns 'Hello, guest!'.

❓ Predict Output

advanced

Nullable type with strict type enforcement

What happens when this PHP code runs?

PHP

<?php
declare(strict_types=1);
function multiply(?int $a, int $b): int {
    return ($a ?? 1) * $b;
}
echo multiply(null, 5);

A TypeError

B 5

C 0

D null

💡 Hint

The null coalescing operator replaces null with 1 before multiplication.

Since $a is null, ($a ?? 1) becomes 1, so 1 * 5 = 5 is returned and printed.

❓ Predict Output

expert

Nullable type with union and default

What is the output of this PHP code?

PHP

<?php
function format(?string|int $value = null): string {
    return match (true) {
        is_int($value) => "Number: $value",
        is_string($value) => "Text: $value",
        default => "No value",
    };
}
echo format();

A Number: 0

B TypeError

C No value

D Text:

💡 Hint

The default parameter is null, which matches the default case in match.

Since $value is null, none of the first two conditions match, so 'No value' is returned.

Practice

1. What does a nullable type declaration like ?int mean in a PHP function parameter?

easy

A. The parameter is optional and defaults to zero.

B. The parameter can be either an integer or null.

C. The parameter can be any type except integer.

D. The parameter must always be an integer, never null.

Solution

1. Step 1: Understand nullable type syntax

   The ? before int means the parameter can accept null or an int value.

2. Step 2: Interpret the meaning in function parameters

   This allows the function to safely accept missing or empty values represented by null.

3. Final Answer:

   The parameter can be either an integer or null. -> Option B

4. Quick Check:

   Nullable type = int or null [OK]

Hint: Nullable means type or null allowed, check for ? before type [OK]

Common Mistakes:

• Thinking nullable means optional parameter
• Assuming nullable excludes null values
• Confusing nullable with default values

2. Which of the following is the correct nullable type syntax to declare a function parameter that accepts a nullable string in PHP?

easy

A. function greet(string? $name) {}

B. function greet(string|null $name) {}

C. function greet(?string $name) {}

D. function greet($name?: string) {}

Solution

1. Step 1: Recall nullable type syntax in PHP

   Nullable types use a question mark ? before the type, like ?string.

2. Step 2: Check each option's syntax

   function greet(?string $name) {} uses ?string correctly. function greet(string|null $name) {} uses union type syntax which is valid in PHP 8+, but the question asks for nullable syntax specifically. Options C and D are invalid syntax.

3. Final Answer:

   function greet(?string $name) {} -> Option C

4. Quick Check:

   Nullable syntax = ?type before parameter [OK]

Hint: Use ? before type for nullable, not after or inside parentheses [OK]

Common Mistakes:

• Placing ? after the type name
• Using union type syntax instead of nullable syntax
• Misplacing the question mark in parameter

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

function checkAge(?int $age) {
    if ($age === null) {
        return "Age not provided";
    }
    return "Age is $age";
}

echo checkAge(null);

medium

A. Age not provided

B. Age is

C. Error: Argument must be int

D. Age is null

Solution

1. Step 1: Understand nullable parameter and null check

   The function accepts an ?int, so null is allowed. It checks if $age === null and returns "Age not provided".

2. Step 2: Call the function with null

   Calling checkAge(null) triggers the null check, so it returns "Age not provided".

3. Final Answer:

   Age not provided -> Option A

4. Quick Check:

   Nullable param null input returns null check message [OK]

Hint: Check for null explicitly when using nullable types [OK]

Common Mistakes:

• Assuming null converts to empty string
• Expecting type error on null input
• Ignoring strict null check with ===

4. Identify the error in this PHP function declaration:

function setName(?string name) {
    echo $name;
}

medium

A. Missing dollar sign ($) before parameter name.

B. Nullable types cannot be used with strings.

C. Function must return a value.

D. Parameter cannot be nullable without default value.

Solution

1. Step 1: Check parameter syntax

   In PHP, all variable names must start with a dollar sign $. The parameter name is missing $.

2. Step 2: Validate nullable type usage

   Nullable types like ?string are valid. The function does not require a return value. Parameters can be nullable without default values.

3. Final Answer:

   Missing dollar sign ($) before parameter name. -> Option A

4. Quick Check:

   Parameter variables need $ prefix [OK]

Hint: Always prefix parameter names with $ in PHP functions [OK]

Common Mistakes:

• Omitting $ in parameter names
• Thinking nullable types need default values
• Expecting functions must return values

5. You want to write a PHP function that accepts a nullable float and returns its square if given, or returns 0 if null. Which of the following implementations is correct?

hard

A. function square(?float $num): float {
return $num * $num;
}

B. function square(?float $num): ?float {
return $num ? $num * $num : 0;
}

C. function square(float $num = null): float {
return $num * $num;
}

D. function square(?float $num): float {
if ($num === null) {
return 0;
}
return $num ** 2;
}

Solution

1. Step 1: Check nullable parameter and return type

   The function accepts ?float and returns float. It must handle null safely and return a float.

2. Step 2: Analyze each option's logic

   function square(?float $num): float { return $num * $num; } does not check for null and will cause an error if $num is null. function square(?float $num): float { if ($num === null) { return 0; } return $num ** 2; } checks for null explicitly and returns 0, then squares the number correctly. function square(float $num = null): float { return $num * $num; } uses default null but parameter is not nullable, causing a type error. function square(?float $num): ?float { return $num ? $num * $num : 0; } returns nullable float but uses a loose check $num ? which fails for 0.0 (falsey), so it returns 0 incorrectly.

3. Final Answer:

   function square(?float $num): float { if ($num === null) { return 0; } return $num ** 2; } -> Option D

4. Quick Check:

   Check null explicitly, return float, use strict null check [OK]

Hint: Always check null explicitly before math with nullable types [OK]

Common Mistakes:

• Not checking for null before calculation
• Using loose truthy checks that fail on 0.0
• Mismatching return types with nullable parameters