🧮 PHP Functions – Reusable Logic and Functional Programming in PHP

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🧲 Introduction – Why Learn PHP Functions?

Functions are the core building blocks of any PHP application. They allow you to encapsulate code into reusable blocks, improve readability, reduce repetition, and help organize complex logic.

PHP supports a wide variety of function-related features—from basic parameter passing to advanced concepts like closures, arrow functions, and recursion.

🎯 In this guide, you’ll learn how to:

• Define, call, and pass parameters to functions
• Use default, named, and variable arguments
• Handle return values, type hints, recursion, closures, and anonymous functions
• Understand variable scope and functional programming in PHP

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📘 Topics Covered

🔖 Topic	📄 Description
🔀 PHP Function Parameters	Accept values inside functions
➡️ Call by Value / Reference	Choose how to pass arguments
🧰 Default Arguments	Use fallback values in functions
🏷️ Named Arguments	Call arguments by name (PHP 8+)
🔢 Variable Arguments	Accept an arbitrary number of parameters
🎯 Returning Values	Send output back from functions
🔁 Passing Functions	Pass functions as arguments
🔄 Recursive Functions	Call a function from itself
📌 Type Hints	Declare types for arguments and return values
🧱 Variable Scope	Understand function-level variable access
🌍 Local & Global Variables	Control access to variables inside/outside functions
🕶️ Anonymous Functions	Create functions without names
🎯 Arrow Functions	Use short syntax for inline functions
🔧 Variable Functions	Call functions using variable names
⚙️ Closure::call()	Bind closures to objects dynamically

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🔀 PHP Function Parameters – Accepting Inputs

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

✅ Parameters are values passed to a function when it’s called.

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➡️ Call by Value / Reference

// By value
function increment($num) {
  $num++;
}
// By reference
function incrementRef(&$num) {
  $num++;
}

✅ Use & to pass by reference and modify original variable.

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🧰 Default Arguments

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

✅ Use default values when no argument is passed.

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🏷️ Named Arguments (PHP 8+)

function profile($name, $age) {
  echo "$name is $age years old.";
}
profile(age: 30, name: "Bob");

✅ Allow calling functions with arguments in any order by name.

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🔢 Variable Arguments

function total(...$numbers) {
  return array_sum($numbers);
}
echo total(1, 2, 3); // Outputs: 6

✅ Use ... to accept multiple arguments as an array.

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🎯 Returning Values

function add($a, $b) {
  return $a + $b;
}
$result = add(5, 3);

✅ Return values using the return statement.

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🔁 Passing Functions – First-Class Functions

function sayHello($name) {
  return "Hello, $name";
}

function greetUser($callback, $name) {
  return $callback($name);
}

echo greetUser("sayHello", "John");

✅ Functions can be passed as strings or anonymous closures.

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🔄 Recursive Functions – Self-Calling

function factorial($n) {
  if ($n <= 1) return 1;
  return $n * factorial($n - 1);
}
echo factorial(5); // Outputs: 120

✅ Useful for tree structures, algorithms, and problems like factorial.

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📌 PHP Type Hints – Declare Expected Types

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

✅ Improves code safety by enforcing data types.

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🧱 Variable Scope

• Local: Defined inside a function
• Global: Defined outside and accessed using global or $GLOBALS

$site = "PHP Site";
function showSite() {
  global $site;
  echo $site;
}

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🌍 Local & Global Variables

$globalVar = 5;
function test() {
  global $globalVar;
  $globalVar++;
}

✅ Use global keyword to access variables outside the function.

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🕶️ Anonymous Functions – No Name, Just Action

$greet = function($name) {
  return "Hello, $name";
};
echo $greet("Eva");

✅ Great for callbacks or dynamic behavior.

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🎯 Arrow Functions – Short Anonymous Syntax

$square = fn($n) => $n * $n;
echo $square(4); // 16

✅ Automatically inherit variables from the parent scope.

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🔧 Variable Functions – Dynamic Function Names

function greet() {
  echo "Hi!";
}
$func = "greet";
$func(); // Hi!

✅ Call functions using variable names.

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⚙️ Closure::call() – Bind Closure to Object

class Person {
  private $name = "John";
}

$getName = function() {
  return $this->name;
};

echo $getName->call(new Person());

✅ Allows closures to access private/protected properties dynamically.

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📘 Best Practices for PHP Functions

✅ Do This	❌ Avoid This
Use meaningful names	Avoid cryptic names like f1()
Keep functions short & focused	Don’t cram multiple tasks into one function
Type-hint arguments & return types	Avoid loosely typed parameters
Use default values & named args	Don’t assume all arguments are always passed
Reuse logic with recursion or callbacks	Avoid repeated code blocks

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📌 Summary – Recap & Next Steps

Functions make your PHP code modular, reusable, and readable. Understanding how to define, pass, and call functions—along with advanced topics like recursion, closures, and arrow functions—equips you to write efficient and modern PHP code.

🔍 Key Takeaways:

• Define functions with parameters, return types, and type hints
• Use recursion, anonymous functions, and variable arguments smartly
• Handle variable scope and leverage closures for advanced tasks

⚙️ Real-World Relevance:
Used for API logic, validation, processing forms, utility functions, configuration, and more.

➡️ Next Up: Explore 🧱 PHP Object-Oriented Programming – Classes, Objects, and Inheritance

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❓ FAQs – PHP Functions

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❓ What’s the difference between return and echo?
✅ return sends the value back to the caller, echo outputs directly to the browser.

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❓ Can I pass a function as a parameter in PHP?
✅ Yes! Use function names as strings or anonymous functions/closures.

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❓ What are arrow functions in PHP?
✅ A short syntax for anonymous functions that inherit variables automatically.

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❓ Can I define a function inside another function in PHP?
✅ Yes, but it will only be available after the outer function is called.

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❓ How do I enforce data types in PHP functions?
✅ Use type hints for parameters and return types, and enable declare(strict_types=1).

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