PHP Functions - Beginner Lesson

What is a Function? (Function)

Concept

Function is a named block of code that you can call (execute) many times. Think of it as a reusable recipe.

Why use Functions?

Reusability: Write once, call many times
Readability: Organized, easy to understand
Maintainability: Fix one place, affects everywhere

Code

<?php
// Define a function
function sayHello() {
    echo "Hello! Welcome to PHP Functions!";
}

// Call the function
sayHello();  // Output: Hello! Welcome to PHP Functions!
sayHello();  // Can call many times!
?>

Output

Call 1: Hello! Welcome to PHP Functions!
Call 2: Hello! Welcome to PHP Functions!

User-defined Functions

Concept

You create your own function using the function keyword, followed by a name and { body }.

Syntax:

function functionName($param1, $param2) { // body }

Code

<?php
// Calculate rectangle area
function calculateArea(float $width, float $height): float {
    return $width * $height;
}

// Welcome message
function welcomeStudent(string $name, int $year): string {
    return "Welcome $name! You are Year $year student.";
}

echo calculateArea(5, 3);         // 15
echo welcomeStudent("Sokha", 2);
?>

Output

calculateArea(5, 3) = 15
Welcome Sokha! You are Year 2 student.

Built-in Functions (PHP has hundreds!)

Concept

PHP provides hundreds of ready-made functions. Just call them - no need to write them!

Code + Output

#	Function	Description	Code	Output
1	strlen()	String length	strlen("Hello")	5
2	strtoupper()	Uppercase	strtoupper("php")	PHP
3	substr()	Substring	substr("Functions",0,4)	Func
4	explode()	Split string	explode(",","A,B,C")	A, B, C
5	implode()	Join array	implode("-",["X","Y"])	X-Y-Z
6	count()	Count elements	count([10,20,30])	3
7	array_sum()	Sum of array	array_sum([10,20,30])	60
8	date()	Current date	date("Y-m-d")	2026-02-28

Global Scope vs Local Scope

Concept

Local Scope: Variables created inside a function cannot be used outside.

Global Scope: Variables outside a function cannot be directly used inside a function.

To access global variables inside a function, use global keyword or $GLOBALS array.

Code

<?php
$school = "ABC School";  // Global variable

function showLocal() {
    $local = "I am local";   // Local variable
    echo $local;
    // echo $school;  // ERROR! Cannot access global directly
}

// Method 1: global keyword
function showGlobal1() {
    global $school;
    echo "Method 1 (global): $school";
}

// Method 2: $GLOBALS
function showGlobal2() {
    echo "Method 2 (\$GLOBALS): " . $GLOBALS['school'];
}
?>

Output

Local inside function: I am local variable
Method 1 (global keyword): ABC School
Method 2 ($GLOBALS): ABC School
Warning: $local outside function = undefined (not accessible)

Parameters

5A - Normal Parameters

You must pass values every time you call the function.

<?php
function addScores(int $math, int $science): int {
    return $math + $science;
}
echo addScores(85, 90); // 175
?>

addScores(85, 90) = 175

5B - Default Parameters

If you don't pass a value, the function uses the default value you defined.

<?php
function greet(string $name, string $greeting = "Hello"): string {
    return "$greeting, $name!";
}
echo greet("Dara");              // Hello, Dara!
echo greet("Dara", "Good day");  // Good day, Dara!
?>

greet('Dara') = Hello, Dara!
greet('Dara','Good day') = Good day, Dara!

5C - Type Hints on Parameters

Specify data types before parameters to prevent bugs: int $age, string $name, float $price

<?php
function calcDiscount(float $price, float $percent = 10.0): float {
    return $price - ($price * $percent / 100);
}
echo calcDiscount(100.0);        // 90 (default 10%)
echo calcDiscount(100.0, 25.0);  // 75 (25% off)
?>

calcDiscount(100.0) = 90 (default 10%)
calcDiscount(100.0, 25.0) = 75 (25% off)

Return Values

Concept

return sends a value back from the function to the caller. You can return a single value or an array.

Return Single Value

<?php
function getGrade(int $score): string {
    if ($score >= 85) return "A";
    if ($score >= 70) return "B";
    if ($score >= 50) return "C";
    return "F";
}
echo getGrade(92); // A
echo getGrade(65); // C
?>

getGrade(92) = A
getGrade(65) = C
getGrade(45) = F

Return Array

<?php
function getStudentInfo(): array {
    return [
        'name'  => 'Vuthy',
        'age'   => 20,
        'grade' => 'A'
    ];
}
$student = getStudentInfo();
echo $student['name'];  // Vuthy
?>

Student: name=Vuthy, age=20, grade=A

Passing by Reference

Concept

Pass by Value (default): Function receives a copy. Original variable does NOT change.

Pass by Reference (&): Function receives the actual address. Changes inside = changes outside!

Use & before parameter: function inc(&$x)

Code

<?php
// Pass by Value - original does NOT change
function addTenValue(int $x): int {
    $x += 10;
    return $x;
}

// Pass by Reference - original DOES change
function addTenRef(int &$x): void {
    $x += 10;
}

$num = 5;
$result = addTenValue($num);
echo "After by-value: \$num = $num";   // still 5
echo "Return value: $result";          // 15

$num2 = 5;
addTenRef($num2);
echo "After by-ref: \$num2 = $num2";   // now 15!
?>

Output

By Value: $num = 5 (unchanged), return = 15
By Reference: $num2 = 15 (changed to 15!)

Type Hinting (Parameter Types + Return Types)

Concept

Parameter type hints: Specify what type of data a function expects: int, float, string, array, bool

Return type: Specify what the function returns using : type after the parentheses.

void: Use : void when the function returns nothing.

About declare(strict_types=1):

By default, PHP will try to convert types automatically (e.g., "5" becomes 5). If you add declare(strict_types=1); at the very top of your file, PHP will throw a TypeError if the wrong type is passed. For beginners, it's optional but good practice!

Code

<?php
// Parameter types + return type
function calcAvg(float $a, float $b, float $c): float {
    return ($a + $b + $c) / 3;
}

function formatName(string $first, string $last): string {
    return strtoupper($last) . ", " . $first;
}

function getScoreList(): array {
    return [85, 92, 78, 95];
}

function printInfo(string $msg): void {
    echo $msg;  // void = returns nothing
}

echo calcAvg(80, 90, 70);          // 80
echo formatName("Sokha", "Chan");  // CHAN, Sokha
?>

Output

calcAvg(80,90,70) = 80
formatName('Sokha','Chan') = CHAN, Sokha
getScoreList() = [85, 92, 78, 95]

Anonymous Functions / Closures

Concept

Anonymous Function: A function without a name, often stored in a variable or passed to another function.

Closure: An anonymous function that can capture (use) variables from its parent scope with use ($var).

Common use cases: array_map(), array_filter(), usort()

Code

<?php
// 1) Anonymous function in a variable
$double = function(int $n): int {
    return $n * 2;
};
echo $double(5);  // 10

// 2) array_map with anonymous function
$scores = [75, 82, 90, 68];
$bonused = array_map(function($s) { return $s + 5; }, $scores);
// [80, 87, 95, 73]

// 3) array_filter - keep scores >= 80
$high = array_filter($scores, function($s) { return $s >= 80; });
// [82, 90]

// 4) Closure with use() - capture external variable
$taxRate = 0.10;
$addTax = function(float $price) use ($taxRate): float {
    return $price + ($price * $taxRate);
};
echo $addTax(100);  // 110
?>

Output

$double(5) = 10
array_map (+5 bonus): [80, 87, 95, 73]
array_filter (>=80): [82, 90]
Closure use($taxRate=0.10): addTax(100) = 110

🎮 Interactive Demos

Demo 1: Function Playground (Calculator)

Enter two numbers and choose an operation. The result is computed using user-defined functions.

Demo 2: Scope Demo

Click "Run" to see how global vs local scope works in PHP.

Demo 3: Default Parameters

Enter a name. Leave greeting empty to use the default value ("Hello").

Demo 4: Type Hinting & Return Type

Choose a type and enter a value. For array, enter comma-separated values (e.g. "a,b,c").

Demo 5: Pass by Reference

Enter a number. See the difference between pass-by-value and pass-by-reference after adding 10.

Demo 6: Anonymous Function / Closure

Click "Run" to see array_map, array_filter, usort, and closure use() in action with student scores.

✍️ Practice Exercises (18 Questions)

A. Basic Functions + Return (6 questions)

P1. Write a function square($n) that returns $n * $n.

Hint: Use return $n * $n inside the function.

function square(int $n): int {
    return $n * $n;
}
echo square(4);  // 16
echo square(7);  // 49

P2. Write a function isEven($n) that returns true if $n is even, false otherwise.

Hint: Use $n % 2 === 0.

function isEven(int $n): bool {
    return $n % 2 === 0;
}
echo isEven(4) ? "true" : "false";  // true
echo isEven(7) ? "true" : "false";  // false

P3. Write a function maxOfThree($a, $b, $c) that returns the largest of three numbers.

Hint: Use max() built-in or compare with if statements.

function maxOfThree(int $a, int $b, int $c): int {
    return max($a, $b, $c);
}
echo maxOfThree(10, 25, 18);  // 25

P4. Write a function factorial($n) that returns the factorial of $n (e.g., 5! = 120).

Hint: Use a loop or recursion. factorial(0) = 1.

function factorial(int $n): int {
    $result = 1;
    for ($i = 2; $i <= $n; $i++) {
        $result *= $i;
    }
    return $result;
}
echo factorial(5);  // 120
echo factorial(0);  // 1

P5. Write a function getMinMax(array $arr) that returns an array with 'min' and 'max' keys.

Hint: Use min() and max() built-in functions, return an associative array.

function getMinMax(array $arr): array {
    return ['min' => min($arr), 'max' => max($arr)];
}
$result = getMinMax([5, 2, 9, 1, 7]);
echo "Min: " . $result['min'];  // 1
echo "Max: " . $result['max'];  // 9

P6. Write a function reverseString($str) that returns the reversed string without using strrev().

Hint: Use a for loop from end to start, or str_split + array_reverse + implode.

function reverseString(string $str): string {
    $result = '';
    for ($i = strlen($str) - 1; $i >= 0; $i--) {
        $result .= $str[$i];
    }
    return $result;
}
echo reverseString("Hello");  // olleH

B. Scope + Globals (4 questions)

P7. A variable $color = "blue" is defined outside a function. Write a function that prints $color using the global keyword.

Hint: Add "global $color;" as the first line inside the function.

$color = "blue";
function showColor() {
    global $color;
    echo "Color is: $color";
}
showColor();  // Color is: blue

P8. Rewrite P7 using $GLOBALS instead of the global keyword.

Hint: Use $GLOBALS['color'] inside the function.

$color = "blue";
function showColorGlobals() {
    echo "Color is: " . $GLOBALS['color'];
}
showColorGlobals();  // Color is: blue

P9. Create a global counter $count = 0, and a function incrementCounter() that uses global to increase $count by 1 each time it is called. Call it 3 times and print $count.

Hint: global $count; $count++;

$count = 0;
function incrementCounter() {
    global $count;
    $count++;
}
incrementCounter();
incrementCounter();
incrementCounter();
echo $count;  // 3

P10. What is the output of this code? Explain why.

$x = 10;
function test() {
    echo $x;
}
test();

Hint: Can a function access an outside variable without global?

C. Default Parameters + Type Hinting (4 questions)

P11. Write a function power(int $base, int $exp = 2): int that raises $base to $exp power. Default exponent is 2 (square).

Hint: Use a loop or ** operator. power(3) should return 9.

function power(int $base, int $exp = 2): int {
    return $base ** $exp;
}
echo power(3);     // 9  (3^2)
echo power(2, 5);  // 32 (2^5)

P12. Write a function createTag(string $text, string $tag = "p"): string that wraps $text in an HTML tag.

Hint: return "<$tag>$text</$tag>".

function createTag(string $text, string $tag = "p"): string {
    return "<$tag>$text";
}
echo createTag("Hello");          // Hello
echo createTag("Title", "h1");    // Title

P13. Write a function calcBMI(float $weight, float $height): float with return type float. BMI = weight / (height * height).

Hint: Weight in kg, height in meters. Return rounded to 1 decimal.

function calcBMI(float $weight, float $height): float {
    return round($weight / ($height * $height), 1);
}
echo calcBMI(70, 1.75);  // 22.9

P14. Write a function sumArray(array $numbers): int that takes an array of integers and returns their sum. Do NOT use array_sum().

Hint: Use a foreach loop and accumulate the total.

function sumArray(array $numbers): int {
    $total = 0;
    foreach ($numbers as $n) {
        $total += $n;
    }
    return $total;
}
echo sumArray([10, 20, 30, 40]);  // 100

D. Passing by Reference (2 questions)

P15. Write a function doubleIt(&$value) that doubles the value of a variable by reference. Demonstrate that the original variable changes.

Hint: $value *= 2; inside the function. No return needed.

function doubleIt(int &$value): void {
    $value *= 2;
}
$num = 8;
echo "Before: $num\n";  // 8
doubleIt($num);
echo "After: $num\n";   // 16

P16. Write a function swapValues(&$a, &$b) that swaps two values by reference. After calling swapValues($x, $y), $x and $y should be exchanged.

Hint: Use a $temp variable to hold one value during the swap.

function swapValues(mixed &$a, mixed &$b): void {
    $temp = $a;
    $a = $b;
    $b = $temp;
}
$x = "hello";
$y = "world";
swapValues($x, $y);
echo "$x, $y";  // world, hello

E. Anonymous Functions / Closures (2 questions)

P17. Given $numbers = [3, 1, 4, 1, 5, 9], use array_filter() with an anonymous function to keep only numbers greater than 3.

Hint: array_filter($numbers, function($n) { ... });

$numbers = [3, 1, 4, 1, 5, 9];
$filtered = array_filter($numbers, function($n) {
    return $n > 3;
});
print_r($filtered);
// [4, 5, 9]

P18. Create a closure that takes a price and applies a tax rate captured with use($rate). The tax rate is $rate = 0.10 (10%). Calculate the final price (price + tax) for $50 and $120.

Hint: $calcTax = function($price) use ($rate) { ... };

$rate = 0.10;
$calcTax = function(float $price) use ($rate): float {
    return $price + ($price * $rate);
};
echo $calcTax(50);   // 55
echo $calcTax(120);  // 132

🔑 Answer Key (Click to expand/collapse)

All 18 answers in one place

A. Basic Functions + Return (P1-P6)

P1. square($n)

function square(int $n): int { return $n * $n; }
echo square(4);  // 16

P2. isEven($n)

function isEven(int $n): bool { return $n % 2 === 0; }
echo isEven(4) ? "true" : "false";  // true

P3. maxOfThree($a, $b, $c)

function maxOfThree(int $a, int $b, int $c): int { return max($a, $b, $c); }
echo maxOfThree(10, 25, 18);  // 25

P4. factorial($n)

function factorial(int $n): int {
    $result = 1;
    for ($i = 2; $i <= $n; $i++) { $result *= $i; }
    return $result;
}
echo factorial(5);  // 120

P5. getMinMax(array $arr)

function getMinMax(array $arr): array {
    return ['min' => min($arr), 'max' => max($arr)];
}
$r = getMinMax([5,2,9,1,7]);
echo "Min:{$r['min']}, Max:{$r['max']}";  // Min:1, Max:9

P6. reverseString($str)

function reverseString(string $str): string {
    $result = '';
    for ($i = strlen($str)-1; $i >= 0; $i--) { $result .= $str[$i]; }
    return $result;
}
echo reverseString("Hello");  // olleH

B. Scope + Globals (P7-P10)

P7. Use global keyword

$color = "blue";
function showColor() { global $color; echo "Color: $color"; }
showColor();  // Color: blue

P8. Use $GLOBALS

$color = "blue";
function showColorG() { echo "Color: " . $GLOBALS['color']; }
showColorG();  // Color: blue

P9. Global counter

$count = 0;
function inc() { global $count; $count++; }
inc(); inc(); inc();
echo $count;  // 3

P10. Scope quiz

Output: Warning (undefined variable). $x inside test() is not the same as global $x. Need global $x; to access it.

C. Default Params + Type Hinting (P11-P14)

P11. power($base, $exp=2)

function power(int $base, int $exp = 2): int { return $base ** $exp; }
echo power(3);     // 9
echo power(2, 5);  // 32

P12. createTag($text, $tag="p")

function createTag(string $text, string $tag = "p"): string {
    return "<$tag>$text";
}
echo createTag("Hello");        // <p>Hello</p>
echo createTag("Title", "h1");  // <h1>Title</h1>

P13. calcBMI($weight, $height)

function calcBMI(float $w, float $h): float {
    return round($w / ($h * $h), 1);
}
echo calcBMI(70, 1.75);  // 22.9

P14. sumArray(array $numbers)

function sumArray(array $numbers): int {
    $total = 0;
    foreach ($numbers as $n) { $total += $n; }
    return $total;
}
echo sumArray([10, 20, 30, 40]);  // 100

D. Passing by Reference (P15-P16)

P15. doubleIt(&$value)

function doubleIt(int &$value): void { $value *= 2; }
$num = 8;
doubleIt($num);
echo $num;  // 16

P16. swapValues(&$a, &$b)

function swapValues(mixed &$a, mixed &$b): void {
    $temp = $a; $a = $b; $b = $temp;
}
$x = "hello"; $y = "world";
swapValues($x, $y);
echo "$x, $y";  // world, hello

E. Anonymous Functions / Closures (P17-P18)

P17. array_filter with anonymous function

$numbers = [3, 1, 4, 1, 5, 9];
$filtered = array_filter($numbers, function($n) { return $n > 3; });
print_r($filtered);  // [4, 5, 9]

P18. Closure with use($rate)

$rate = 0.10;
$calcTax = function(float $price) use ($rate): float {
    return $price + ($price * $rate);
};
echo $calcTax(50);   // 55
echo $calcTax(120);  // 132