Functions are reusable blocks of code that perform a specific task.

👉 Instead of writing the same code again and again, you define a function and call it when needed.

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1️⃣ User-defined Functions

These are functions created by you using the function keyword.

✅ Syntax:

function functionName() {
    // code
}

✅ Example:

function sayHello() {
    echo "Hello World!";
}sayHello(); // calling function

👉 Output:

Hello World!

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2️⃣ Function Parameters

Parameters are inputs passed to a function.

✅ Example:

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

👉 Output:

Hello Aditya

👉 You can pass multiple parameters:

function add($a, $b) {
    echo $a + $b;
}add(5, 3);

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3️⃣ Default Arguments

You can assign default values to parameters.

👉 If no value is passed, the default is used.

✅ Example:

function greet($name = "Guest") {
    echo "Hello $name";
}greet();        // uses default
greet("Aditya");

👉 Output:

Hello Guest
Hello Aditya

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4️⃣ Return Values

Functions can return values using return.

👉 This is very important for real applications.

✅ Example:

function add($a, $b) {
    return $a + $b;
}$result = add(10, 5);
echo $result;

👉 Output:

15

👉 Difference:

• echo → directly prints
• return → sends value back

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5️⃣ Variable Functions

In PHP, you can call a function using a variable name.

👉 Function name stored in a variable.

✅ Example:

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

👉 Output:

Hi!

👉 Useful in:

• Dynamic function calls
• Callback systems

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6️⃣ Recursive Functions

A function that calls itself is called a recursive function.

👉 Used for:

• Factorial
• Tree structures
• File systems

✅ Example: Factorial

function factorial($n) {
    if ($n == 0) {
        return 1;
    }
    return $n * factorial($n - 1);
}echo factorial(5);

👉 Output:

120

👉 Flow:

5 × 4 × 3 × 2 × 1

⚠️ Important:
Always define a base condition, otherwise infinite loop will occur.

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🎯 Summary

Concept	Meaning
User-defined	Custom functions created by user
Parameters	Inputs passed to function
Default arguments	Predefined values
Return values	Output from function
Variable functions	Call function using variable
Recursive functions	Function calling itself

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🧠 Practice Questions

Try these:

1. Create a function to check even/odd
2. Function to calculate square of a number
3. Function with default argument (country = India)
4. Return largest of two numbers
5. Create a recursive function for sum of numbers (1 to n)
6. Use variable function to call different math operations