PHP: Functions — The Basics

Q: When should I use the void return type?

When a function performs an action without returning data — e.g., outputting a page, writing a log, sending an email, modifying global state. void tells callers: "This function does something — don't expect a return value."

Q: What's the difference between type declarations and type conversion?

Type declarations in the function signature say "I expect this type." PHP automatically coerces types at call time. Enable strict_types to disable auto-coercion — passing the wrong type then causes an error, which is safer.

Q: Can I write code after return ?

You can, but it won't execute. return immediately ends the function. Your IDE will typically warn you about "unreachable code."

Functions are the basic building blocks of programming. Wrap reusable code into functions, and you level up from "writing scripts" to "building with blocks" — each function becomes a reusable piece.

1. Defining and Calling Functions

PHP

<?php
// Define a function
function greet() {
    echo "Hello, World!";
}

// Call the function
greet();  // Output: Hello, World!
greet();  // You can call it repeatedly
?>

Function names are case-insensitive (but the convention is lowercase with underscores — snake_case):

PHP

GREET();  // This also works, but don't do it

2. Parameters and Return Values

Functions can receive parameters (input) and return data (output):

PHP

<?php
// A function with a parameter
function greet($name) {
    return "Hello, {$name}!";
}

$msg = greet("John");
echo $msg;  // Hello, John!
?>

(1) Multiple Parameters

PHP

<?php
function calculateTotal($price, $quantity, $tax = 1.0) {
    return $price * $quantity * $tax;
}

echo calculateTotal(10, 3);        // 30 (tax uses the default of 1.0)
echo calculateTotal(10, 3, 1.13);  // 33.9 (custom tax passed in)
?>

💡 Tip: Parameters with default values must come after parameters without default values. function f($a = 1, $b) will cause an error.

3. Type Declarations (PHP 7+)

PHP 7+ supports type declarations for function parameters and return values, making your code safer:

PHP

<?php
// Parameter type declarations + return type declaration
function add(int $a, int $b): int {
    return $a + $b;
}

echo add(3, 5);      // 8
echo add("3", "5");  // 8 (PHP auto-converts)
// echo add("hello", 5); // TypeError!

// Other type declarations
function displayUser(string $name, int $age): void {
    echo "{$name}, {$age} years old";
}

function getPrice(float $price): string {
    return "$" . number_format($price, 2);
}
?>

Type	Declaration	Notes
Integer	`int`	Auto-converts to int
Float	`float`	Auto-converts to float
String	`string`	Auto-converts to string
Boolean	`bool`	Auto-converts to bool
Array	`array`	Must be an array
No return	`void`	Cannot return anything
Nullable	`?string`	Accepts string or null
Union type (PHP 8+)	`int\|float`	Accepts int or float
Mixed	`mixed`	Any type

💡 Tip: When writing new code, always add type declarations. They make your code's intent clearer and catch bugs early — if the caller passes the wrong type, you get a TypeError instead of silently producing strange results.

▶ Example: Practical Function with Type Declarations

Output:

TEXT 📖 Display only

[" . date("H:i:s") . "] {root} performed {value}<br>
9)
result

PHP

<?php
// Combined type declarations: union type + void + default value
function formatPrice(int|float $price, string $currency = "$"): string {
    return $currency . number_format((float)$price, 2);
}

function logAction(string $user, string $action): void {
    echo "[" . date("H:i:s") . "] {$user} performed {$action}<br>";
}

echo formatPrice(39.9);         // $39.90
echo formatPrice(100);           // $100.00
logAction("admin", "logged in"); // [14:30:45] admin performed logged in
?>

Output:

TEXT 📖 Display only

Output displayed

4. Strict Typing (strict_types)

By default, PHP's type declarations allow automatic type coercion ("3" can be passed to an int parameter). Enable strict mode and the types must match exactly:

▶ Example: Default Mode vs. Strict Mode

Output:

TEXT 📖 Display only

result
result
result
result

PHP

<?php
// Default mode: automatic type coercion
function double(int $n): int {
    return $n * 2;
}
echo double("5");  // 10 ("5" is auto-converted to int 5)

// Strict mode: by the book
declare(strict_types=1);

function doubleStrict(int $n): int {
    return $n * 2;
}
echo doubleStrict("5");  // TypeError! Must pass an int
echo doubleStrict(5);    // 10 ✅

function greetStrict(string $name): string {
    return "Hello, {$name}";
}
// echo greetStrict(123);  // TypeError! A number is not a string
echo greetStrict("John");  // ✅
?>

Output:

TEXT 📖 Display only

Output displayed

💡 Tip: declare(strict_types=1) must be the first statement in a PHP file (immediately after the <?php tag). It only affects function calls within the current file, not the file where the function is defined. For new projects, enable strict mode by default.

5. The return Statement

PHP

<?php
function getUser($id) {
    // Query the database... let's pretend we found a user
    if ($id <= 0) {
        return null;  // Invalid ID — bail out early
    }
    return [
        "id" => $id,
        "name" => "User{$id}"
    ];
}

$user = getUser(1);
if ($user !== null) {
    echo $user["name"];  // User1
}

// void function: returns nothing
function logError(string $message): void {
    // Write to the error log
    error_log($message);
    // Do not write return $something;
}
?>

After return, the function ends immediately. This is especially useful for guard clauses — handling exceptional cases first and returning early.

6. PHPDoc Comments

Good functions need documentation. PHPDoc is PHP's comment standard:

PHP

<?php
/**
 * Calculate the total price
 *
 * @param float $price    Unit price
 * @param int   $quantity Quantity purchased
 * @param float $tax      Tax rate (default 1.0 for tax-inclusive)
 * @return float Final total price
 *
 * Examples:
 *   calculateTotal(10, 3)       → 30.0
 *   calculateTotal(10, 3, 1.13) → 33.9
 */
function calculateTotal(float $price, int $quantity, float $tax = 1.0): float {
    return $price * $quantity * $tax;
}
?>

Tag	Purpose
`@param`	Parameter description (type + name + explanation)
`@return`	Return value description
`@throws`	Exceptions that may be thrown
`@var`	Variable type
`@see`	Related reference

💡 Tip: PHPDoc isn't just for humans — VS Code's Intelephense extension reads it to provide autocomplete and type hints. Well-written PHPDoc turns your IDE into an intelligent assistant.

7. Function Naming Conventions

PHP function names follow the snake_case community standard:

PHP

// ✅ Good function names
function get_user_by_id($id) {}
function calculate_total_price($items) {}
function send_verification_email($email) {}

// ❌ Inconsistent naming
function GetUserById($id) {}    // Don't use PascalCase
function getUserById($id) {}    // Don't use camelCase (methods use this, not functions)
function getuserbyid($id) {}    // Don't use all-lowercase-no-separators

💡 Tip: Verb + noun: get_ (fetch), set_ (assign), calculate_ (compute), create_ (make), delete_ (remove), validate_ (check), format_ (present).

▶ Example: Default Parameters and Nullable Types

PHP

<?php
function greet(string $name, string $greeting = "Hello", ?string $title = null): string {
    $prefix = $title !== null ? $title . " " : "";
    return "{$greeting}, {$prefix}{$name}!";
}

echo greet("Alice");                    // Hello, Alice!
echo greet("Bob", "Hi");               // Hi, Bob!
echo greet("Carol", "Welcome", "Dr."); // Welcome, Dr. Carol!

▶ Try it Yourself

Output:

TEXT 📖 Display only

Output displayed

❓ FAQ

Q When should I use the void return type?

A When a function performs an action without returning data — e.g., outputting a page, writing a log, sending an email, modifying global state. void tells callers: "This function does something — don't expect a return value."

Q What's the difference between type declarations and type conversion?

A Type declarations in the function signature say "I expect this type." PHP automatically coerces types at call time. Enable strict_types to disable auto-coercion — passing the wrong type then causes an error, which is safer.

Q Can I write code after return?

A You can, but it won't execute. return immediately ends the function. Your IDE will typically warn you about "unreachable code."

📖 Summary

function name($param): type { return ... } defines a function
Parameters can have default values (must come after required parameters)
Type declarations (int, string, ?string, int|float) make code safer
declare(strict_types=1) enables strict mode — no auto type coercion
void means no return value; return immediately ends the function
Function names use snake_case — verb + noun, like get_user_by_id
PHPDoc comments are for both humans and your IDE

📝 Exercises

Write a function calculateBMI($weight, $height) that takes weight (kg) and height (m), returns the BMI (weight ÷ height²), and appends an evaluation like "Underweight / Normal / Overweight / Obese."
Enable strict mode and write a function formatCurrency(float $amount, string $symbol = "$"): string that formats a monetary amount. Use strict mode to ensure the amount is always a float.
Write a function getPageUrl(string $base, int $page): string that builds a pagination URL. If page is 1, return just the base; otherwise append ?page=N. Write PHPDoc comments for it.

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PHP: Functions — The Basics

1. Defining and Calling Functions

2. Parameters and Return Values

(1) Multiple Parameters

3. Type Declarations (PHP 7+)

▶ Example: Practical Function with Type Declarations

4. Strict Typing (strict_types)

▶ Example: Default Mode vs. Strict Mode

5. The return Statement

6. PHPDoc Comments

7. Function Naming Conventions

▶ Example: Default Parameters and Nullable Types

❓ FAQ

📖 Summary

📝 Exercises

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