Union types in practice in PHP - Mini Project: Build & Apply

Union types in practice

📖 Scenario: You are building a simple PHP program to handle user input that can be either a string or an integer. This is common when dealing with form data where users might enter numbers or text.

🎯 Goal: Create a PHP function that accepts a parameter which can be either an int or a string using union types. The function will return a message describing the type and value of the input.

📋 What You'll Learn

Create a function called describeInput that accepts a parameter with a union type int|string.

Inside the function, use is_int() and is_string() to check the type of the input.

Return a string message describing the input type and value.

Call the function with both an integer and a string and print the results.

💡 Why This Matters

🌍 Real World

Handling user input that can be multiple types is common in web forms and APIs. Union types help make your code clear and safe.

💼 Career

Understanding union types is important for writing modern PHP code that is robust and easy to maintain, a skill valued in backend development jobs.

1

Create the describeInput function with union type parameter

Write a PHP function called describeInput that accepts one parameter named $input with the union type int|string. Inside the function, just return an empty string "" for now.

PHP

<?php

function describeInput(int|string $input): string {
    // Your code here
}

Hint

Use the syntax function describeInput(int|string $input): string to declare the function with a union type parameter.

2

Add type checks inside describeInput

Inside the describeInput function, add an if statement to check if $input is an integer using is_int($input). If true, return the string "Input is an integer: " concatenated with the value of $input. Add an elseif to check if $input is a string using is_string($input) and return "Input is a string: " concatenated with $input. Keep the function signature and the empty return removed.

PHP

<?php

function describeInput(int|string $input): string {
    // Your code here
}

Hint

Use is_int() and is_string() to check the type and return the appropriate message.

3

Call describeInput with an integer and a string

After the function, write two lines of code that call describeInput with the integer 42 and the string "hello". Store the results in variables $resultInt and $resultStr respectively.

PHP

<?php

function describeInput(int|string $input): string {
    if (is_int($input)) {
        return "Input is an integer: " . $input;
    } elseif (is_string($input)) {
        return "Input is a string: " . $input;
    }
}

// Your code here

Hint

Call the function with 42 and "hello" and save the results in $resultInt and $resultStr.

4

Print the results

Print the variables $resultInt and $resultStr each on its own line using echo. Use "\n" for new lines.

PHP

<?php

function describeInput(int|string $input): string {
    if (is_int($input)) {
        return "Input is an integer: " . $input;
    } elseif (is_string($input)) {
        return "Input is a string: " . $input;
    }
}

$resultInt = describeInput(42);
$resultStr = describeInput("hello");

// Your code here

Hint

Use echo $resultInt . "\n"; and echo $resultStr . "\n"; to print each result on a new line.

Practice

1. What does a union type declaration like int|string mean in PHP?

easy

A. The value must be both an integer and a string at the same time.

B. The value can only be a string.

C. The value can only be an integer.

D. The value can be either an integer or a string.

Solution

1. Step 1: Understand union type syntax

   The | symbol means the value can be one of the listed types.

2. Step 2: Interpret int|string

   This means the value can be either an integer or a string, not both simultaneously.

3. Final Answer:

   The value can be either an integer or a string. -> Option D

4. Quick Check:

   Union type int|string means either int or string [OK]

Hint: Union types allow multiple possible types separated by | [OK]

Common Mistakes:

• Thinking the value must be both types at once
• Confusing union with intersection types
• Assuming only one type is allowed

2. Which of the following is the correct syntax for an optional function parameter that accepts either an array or null using union types?

easy

A. function example(array|null $data = null) {}

B. function example(array&null $data) {}

C. function example(array? $data) {}

D. function example(array|null $data) {}

Solution

1. Step 1: Identify correct union type syntax

   Union types use | between types, so array|null is correct.

2. Step 2: Check default value for nullable parameter

   To accept null, the parameter should default to null, so $data = null is needed.

3. Final Answer:

   function example(array|null $data = null) {} -> Option A

4. Quick Check:

   Union type with default null = correct syntax [OK]

Hint: Use | for union and default null for optional nullable [OK]

Common Mistakes:

• Using & instead of | for union
• Missing default null for nullable parameter
• Using ? without type for union

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

function describe(mixed $value): string {
    return match (gettype($value)) {
        'integer' => 'Number: ' . $value,
        'string' => 'Text: ' . $value,
        default => 'Unknown type'
    };
}

echo describe(10) . ', ' . describe('hello') . ', ' . describe(3.14);

medium

A. Number: 10, Text: hello, Unknown type

B. Number: 10, Text: hello, Number: 3.14

C. Unknown type, Text: hello, Unknown type

D. Number: 10, Unknown type, Unknown type

Solution

1. Step 1: Analyze the match expression with gettype()

   For 10, gettype returns 'integer', so output is 'Number: 10'. For 'hello', gettype returns 'string', so output is 'Text: hello'. For 3.14, gettype returns 'double', which matches default case 'Unknown type'.

2. Step 2: Combine outputs

   Outputs joined by commas: 'Number: 10, Text: hello, Unknown type'.

3. Final Answer:

   Number: 10, Text: hello, Unknown type -> Option A

4. Quick Check:

   Match on gettype gives expected strings [OK]

Hint: Match gettype results carefully for each input [OK]

Common Mistakes:

• Assuming float is treated as integer
• Ignoring default case in match
• Confusing gettype return values

4. Identify the error in this PHP function using union types:

function process(int|string $input): int {
    if (is_string($input)) {
        return strlen($input);
    } else {
        return $input + 1;
    }
}

process(true);

medium

A. The is_string check is incorrect syntax.

B. The function should return string, not int.

C. Passing a boolean to a parameter expecting int|string causes a TypeError.

D. The function does not handle float inputs.

Solution

1. Step 1: Check parameter type declaration

   The function expects int|string, so only integers or strings are allowed.

2. Step 2: Analyze the call with true

   Passing true (boolean) violates the declared union type, causing a TypeError at runtime.

3. Final Answer:

   Passing a boolean to a parameter expecting int|string causes a TypeError. -> Option C

4. Quick Check:

   Type mismatch causes error [OK]

Hint: Check argument types match union types exactly [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking is_string is invalid syntax
• Assuming function return type is wrong

5. You want to write a function that accepts either a string or an array of strings and returns a single string. Which of the following implementations correctly uses union types and handles both cases?

hard

A. function joinStrings(string|array $input): string {
return implode(', ', $input);
}

B. function joinStrings(string|array $input): string {
if (is_array($input)) {
return implode(', ', $input);
}
return $input;
}

C. function joinStrings(string|array $input): string {
if (is_string($input)) {
return implode(', ', $input);
}
return $input;
}

D. function joinStrings(array|string $input): string {
if (is_array($input)) {
return $input;
}
return implode(', ', $input);
}

Solution

1. Step 1: Check union type declaration

   Both string|array and array|string are valid syntax.

2. Step 2: Verify handling of each type

   function joinStrings(string|array $input): string { if (is_array($input)) { return implode(', ', $input); } return $input; } checks if input is array, then joins with implode. If input is string, returns it directly. This correctly handles both cases.

3. Step 3: Identify errors in other options

   function joinStrings(string|array $input): string { return implode(', ', $input); } tries to implode without checking type, causing error if input is string. function joinStrings(string|array $input): string { if (is_string($input)) { return implode(', ', $input); } return $input; } incorrectly calls implode on string input. function joinStrings(array|string $input): string { if (is_array($input)) { return $input; } return implode(', ', $input); } returns array directly without joining.

4. Final Answer:

   function joinStrings(string|array $input): string { if (is_array($input)) { return implode(', ', $input); } return $input; } -> Option B

5. Quick Check:

   Check type then handle accordingly [OK]

Hint: Check type before processing union inputs [OK]

Common Mistakes:

• Calling implode on string input
• Returning array instead of string
• Not checking input type before processing