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In this section, we will delve into how to define and use function parameters and return values in PHP. Understanding these concepts is crucial for writing reusable and modular code.

Function Parameters

What are Function Parameters?

Function parameters are variables that are passed to a function when it is called. They allow you to pass data into your functions, making them more flexible and reusable.

Defining Function Parameters

To define parameters in a function, you list them inside the parentheses in the function declaration.

function greet($name) {
    echo "Hello, $name!";
}

In this example, $name is a parameter of the greet function.

Calling a Function with Parameters

When calling a function with parameters, you provide the arguments (values) that correspond to the parameters.

greet("Alice"); // Output: Hello, Alice!
greet("Bob");   // Output: Hello, Bob!

Default Parameters

You can also define default values for parameters. If no argument is provided, the default value is used.

function greet($name = "Guest") {
    echo "Hello, $name!";
}

greet();        // Output: Hello, Guest!
greet("Alice"); // Output: Hello, Alice!

Multiple Parameters

A function can have multiple parameters, separated by commas.

function add($a, $b) {
    return $a + $b;
}

echo add(2, 3); // Output: 5

Return Values

What are Return Values?

A return value is the value that a function sends back to the part of the program that called it. This allows the function to produce a result that can be used elsewhere in the code.

Returning a Value

To return a value from a function, use the return statement followed by the value you want to return.

function add($a, $b) {
    return $a + $b;
}

$result = add(2, 3);
echo $result; // Output: 5

Returning Multiple Values

PHP does not support returning multiple values directly, but you can return an array containing multiple values.

function calculate($a, $b) {
    $sum = $a + $b;
    $difference = $a - $b;
    return array($sum, $difference);
}

list($sum, $difference) = calculate(5, 3);
echo "Sum: $sum, Difference: $difference"; // Output: Sum: 8, Difference: 2

Practical Examples

Example 1: Function with Parameters and Return Value

function multiply($a, $b) {
    return $a * $b;
}

echo multiply(4, 5); // Output: 20

Example 2: Function with Default Parameters

function greet($name = "Guest", $greeting = "Hello") {
    return "$greeting, $name!";
}

echo greet();                // Output: Hello, Guest!
echo greet("Alice");         // Output: Hello, Alice!
echo greet("Bob", "Hi");     // Output: Hi, Bob!

Example 3: Function Returning an Array

function getUserInfo($id) {
    // Simulate fetching user info from a database
    $user = array(
        "id" => $id,
        "name" => "John Doe",
        "email" => "[email protected]"
    );
    return $user;
}

$userInfo = getUserInfo(1);
echo "Name: " . $userInfo["name"]; // Output: Name: John Doe

Exercises

Exercise 1: Create a Function to Calculate the Area of a Rectangle

Task: Write a function calculateArea that takes the length and width of a rectangle as parameters and returns the area.

function calculateArea($length, $width) {
    // Your code here
}

// Test the function
echo calculateArea(5, 10); // Expected Output: 50

Solution:

function calculateArea($length, $width) {
    return $length * $width;
}

echo calculateArea(5, 10); // Output: 50

Exercise 2: Create a Function to Convert Celsius to Fahrenheit

Task: Write a function celsiusToFahrenheit that takes a temperature in Celsius as a parameter and returns the temperature in Fahrenheit.

function celsiusToFahrenheit($celsius) {
    // Your code here
}

// Test the function
echo celsiusToFahrenheit(0);  // Expected Output: 32
echo celsiusToFahrenheit(100); // Expected Output: 212

Solution:

function celsiusToFahrenheit($celsius) {
    return ($celsius * 9/5) + 32;
}

echo celsiusToFahrenheit(0);   // Output: 32
echo celsiusToFahrenheit(100); // Output: 212

Common Mistakes and Tips

Common Mistakes

Forgetting to Return a Value:
- Ensure you use the return statement to send a value back from the function.
```
function add($a, $b) {
    $sum = $a + $b;
    // Missing return statement
}
```
Incorrect Parameter Order:
- Ensure the order of arguments matches the order of parameters in the function definition.
```
function subtract($a, $b) {
    return $a - $b;
}

echo subtract(5, 10); // Output: -5 (not 5)
```

Tips

Use Descriptive Parameter Names:
- Choose parameter names that clearly describe their purpose.
```
function calculateTotal($price, $quantity) {
    return $price * $quantity;
}
```
Default Parameters for Optional Arguments:
- Use default parameters to make some arguments optional.
```
function greet($name = "Guest") {
    echo "Hello, $name!";
}
```

Conclusion

In this section, we covered how to define and use function parameters and return values in PHP. We learned how to pass data into functions, set default parameter values, and return results from functions. These concepts are fundamental for writing modular and reusable code. In the next section, we will explore variable scope in PHP, which will further enhance your understanding of how variables work within functions.

Function Parameters and Return Values