Nullable types in functions in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a nullable type mean in PHP function parameters?

A nullable type means the function parameter can accept either the specified type or null. It is declared by adding a question mark ? before the type.

beginner

How do you declare a nullable integer parameter in a PHP function?

You declare it by adding a question mark before the type like this: <code>function example(?int $number) { ... }</code>. This means <code>$number</code> can be an integer or <code>null</code>.

intermediate

What happens if you pass null to a function parameter that is not nullable?

PHP will throw a TypeError because the parameter expects a specific type and null is not allowed unless the type is nullable.

intermediate

Can return types in PHP functions be nullable? How?

Yes, return types can be nullable by adding a question mark before the return type. For example: <code>function getValue(): ?string { ... }</code> means the function returns a string or <code>null</code>.

beginner

Why are nullable types useful in PHP functions?

Nullable types allow functions to explicitly accept or return null values, making the code clearer and safer by defining when null is an acceptable value.

How do you declare a nullable string parameter in a PHP function?

A function example(string? $param) {}

B function example(?string $param) {}

C function example(string|null $param) {}

D function example($param: ?string) {}

The correct syntax is to put a question mark before the type: ?string.

What will happen if you pass null to a non-nullable int parameter?

A PHP will throw a TypeError.

B It will work fine.

C The parameter will be converted to 0.

D PHP will ignore the parameter.

Passing null to a non-nullable parameter causes a TypeError in PHP.

Which of these is a valid nullable return type declaration?

A function foo(): array|null {}

B function foo(): array? {}

C function foo(): ?array {}

D function foo(): nullable array {}

The correct syntax for nullable return types is a question mark before the type, like ?array.

Nullable types were introduced in which PHP version?

A PHP 5.6

B PHP 7.4

C PHP 8.0

D PHP 7.1

Nullable types were introduced in PHP 7.1.

What does this function signature mean? function test(?float $value): ?float

A Parameter and return value can be float or null.

B Parameter must be float, return can be null.

C Parameter can be null, return must be float.

D Neither parameter nor return can be null.

Both the parameter and return type are nullable float, meaning they can be float or null.

Explain how to declare a nullable parameter and a nullable return type in a PHP function.

Think about the question mark symbol and where it goes.

Why is it important to use nullable types in PHP functions? Give an example scenario.

Consider when a value might be missing or optional.

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