Loops - PHP Foundations

Why Loops?

📖 Definition

Loop: A control structure that repeats a block of code as long as a specified condition remains true (or for a specified number of iterations).

Imagine you need to display a list of 50 products from a database. Without loops, you'd need 50 nearly identical blocks of code. With a loop, you write the display logic once:

<?php
// Without a loop (terrible!) 😱
echo "<li>" . $products[0] . "</li>";
echo "<li>" . $products[1] . "</li>";
echo "<li>" . $products[2] . "</li>";
// ... 47 more lines ...

// With a loop (much better!) ✅
foreach ($products as $product) {
    echo "<li>$product</li>";
}

PHP provides four types of loops:

flowchart LR A["PHP Loops"] --> B["for"] A --> C["while"] A --> D["do-while"] A --> E["foreach"] B --- B1["Know iteration count"] C --- C1["Condition checked first"] D --- D1["Runs at least once"] E --- E1["Iterate arrays/objects"]

The for Loop

The for loop is best when you know how many times you want to iterate. It packs three pieces into one line: initialization, condition, and update.

Syntax

for (initialization; condition; update) {
    // Code to repeat
}

flowchart TD A["Initialize: $i = 0"] --> B{"Condition: $i < 5?"} B -->|Yes| C["Execute loop body"] C --> D["Update: $i++"] D --> B B -->|No| E["Exit loop"]

Basic Examples

<?php
// Count from 1 to 5
for ($i = 1; $i <= 5; $i++) {
    echo "$i ";
}
// Output: 1 2 3 4 5

// Count down from 10 to 1
for ($i = 10; $i >= 1; $i--) {
    echo "$i ";
}
// Output: 10 9 8 7 6 5 4 3 2 1

// Count by twos
for ($i = 0; $i <= 20; $i += 2) {
    echo "$i ";
}
// Output: 0 2 4 6 8 10 12 14 16 18 20

Practical: Building an HTML Table

<?php
echo "<table>\n";
echo "  <tr><th>#</th><th>Square</th><th>Cube</th></tr>\n";

for ($n = 1; $n <= 10; $n++) {
    $square = $n ** 2;
    $cube = $n ** 3;
    $class = ($n % 2 === 0) ? "even" : "odd";
    echo "  <tr class='$class'>";
    echo "<td>$n</td><td>$square</td><td>$cube</td>";
    echo "</tr>\n";
}

echo "</table>";

Looping Through Strings

<?php
$word = "PHP";

for ($i = 0; $i < strlen($word); $i++) {
    echo "Character $i: $word[$i]\n";
}
// Output:
// Character 0: P
// Character 1: H
// Character 2: P

⚠️ Performance Tip

Don't call functions in the condition part — they run every iteration:

// BAD — strlen() called every iteration
for ($i = 0; $i < strlen($word); $i++) { ... }

// GOOD — calculate once
$length = strlen($word);
for ($i = 0; $i < $length; $i++) { ... }

Multiple Expressions

You can use commas to have multiple initializations or updates:

<?php
// Two counters moving in opposite directions
for ($left = 0, $right = 9; $left < $right; $left++, $right--) {
    echo "L=$left, R=$right\n";
}
// Output:
// L=0, R=9
// L=1, R=8
// L=2, R=7
// L=3, R=6
// L=4, R=5

The while Loop

The while loop is best when you don't know in advance how many iterations you need. It checks the condition before each iteration.

Syntax

while (condition) {
    // Code to repeat
    // Must include something that eventually makes condition false!
}

flowchart TD A["Start"] --> B{"Condition true?"} B -->|Yes| C["Execute loop body"] C --> B B -->|No| D["Exit loop"]

Basic Examples

<?php
// Count down — we know when to stop, but while is still natural here
$countdown = 5;
while ($countdown > 0) {
    echo "$countdown... ";
    $countdown--;
}
echo "Liftoff! 🚀";
// Output: 5... 4... 3... 2... 1... Liftoff! 🚀

Practical: Reading User Input (Simulated)

<?php
// Simulating a menu system
$choices = ["view", "edit", "delete", "quit"]; // Simulated input
$index = 0;

while ($index < count($choices)) {
    $choice = $choices[$index];
    echo "Processing: $choice\n";

    if ($choice === "quit") {
        echo "Goodbye!\n";
        break; // Exit early
    }

    $index++;
}
// Output:
// Processing: view
// Processing: edit
// Processing: delete
// Processing: quit
// Goodbye!

Practical: Halving Until Small Enough

<?php
// How many times can you halve 1000 before reaching less than 1?
$value = 1000;
$halvings = 0;

while ($value >= 1) {
    $value /= 2;
    $halvings++;
}

echo "It took $halvings halvings to get below 1.";
// Output: It took 10 halvings to get below 1.

⚠️ Infinite Loops!

If the condition never becomes false, the loop runs forever. This will crash your script (PHP will hit its max_execution_time and abort).

// DANGER — infinite loop!
$x = 1;
while ($x > 0) {
    echo $x;
    $x++;  // x keeps growing — condition is always true!
}

// Always ensure your loop has a way to end:
// ✅ A counter that reaches its limit
// ✅ A condition that changes inside the loop
// ✅ A break statement that exits

The do-while Loop

The do-while loop is identical to while except it checks the condition after the body runs. This guarantees the body executes at least once.

Syntax

do {
    // Code to repeat (runs at least once)
} while (condition);

flowchart TD A["Start"] --> B["Execute loop body"] B --> C{"Condition true?"} C -->|Yes| B C -->|No| D["Exit loop"]

while vs. do-while

<?php
// while — checks FIRST, body may never run
$x = 10;
while ($x < 5) {
    echo "while: $x\n";  // Never runs! 10 is not < 5
    $x++;
}

// do-while — runs FIRST, then checks
$x = 10;
do {
    echo "do-while: $x\n";  // Runs once! Then condition fails
    $x++;
} while ($x < 5);
// Output: do-while: 10

Practical: Input Validation Pattern

The do-while loop shines for validation — you need to get the input at least once before you can check it:

<?php
// Simulating user input validation
$attempts = ["", "abc", "-5", "42"]; // Simulated inputs
$index = 0;

do {
    $input = $attempts[$index] ?? null;
    $index++;

    if ($input === null) {
        echo "No more input.\n";
        break;
    }

    $isValid = is_numeric($input) && (int)$input > 0;

    if (!$isValid) {
        echo "Invalid input: '$input'. Try again.\n";
    }

} while (!$isValid);

if ($isValid) {
    echo "Accepted: $input\n";
}
// Output:
// Invalid input: ''. Try again.
// Invalid input: 'abc'. Try again.
// Invalid input: '-5'. Try again.
// Accepted: 42

Practical: Generating Unique IDs

<?php
$existingIds = ["usr_001", "usr_002", "usr_003"];

do {
    $newId = "usr_" . str_pad(rand(1, 999), 3, "0", STR_PAD_LEFT);
} while (in_array($newId, $existingIds));

echo "Generated unique ID: $newId";

✅ When to Use do-while

Use do-while when the action must happen before you can evaluate the condition. Common scenarios: input validation, generating values until one meets criteria, menu loops, and retry logic.

The foreach Loop

The foreach loop is PHP's most-used loop. It's purpose-built for iterating over arrays and objects — no counters, no index management, no off-by-one errors.

Syntax: Values Only

foreach ($array as $value) {
    // $value is the current element
}

Syntax: Keys and Values

foreach ($array as $key => $value) {
    // $key is the current key/index
    // $value is the current element
}

Indexed Arrays

<?php
$fruits = ["Apple", "Banana", "Cherry", "Date"];

// Values only
foreach ($fruits as $fruit) {
    echo "🍎 $fruit\n";
}
// Output:
// 🍎 Apple
// 🍎 Banana
// 🍎 Cherry
// 🍎 Date

// With index
foreach ($fruits as $index => $fruit) {
    $num = $index + 1;
    echo "$num. $fruit\n";
}
// Output:
// 1. Apple
// 2. Banana
// 3. Cherry
// 4. Date

Associative Arrays

<?php
$user = [
    "name"  => "Alice",
    "email" => "alice@example.com",
    "role"  => "admin",
    "age"   => 28,
];

foreach ($user as $key => $value) {
    echo ucfirst($key) . ": $value\n";
}
// Output:
// Name: Alice
// Email: alice@example.com
// Role: admin
// Age: 28

Practical: Rendering an HTML List

<?php
$tasks = [
    ["title" => "Buy groceries",    "done" => true],
    ["title" => "Write PHP lesson", "done" => false],
    ["title" => "Walk the dog",     "done" => true],
    ["title" => "Fix login bug",    "done" => false],
];

echo "<ul class='task-list'>\n";
foreach ($tasks as $task) {
    $icon = $task["done"] ? "✅" : "⬜";
    $class = $task["done"] ? "completed" : "pending";
    echo "  <li class='$class'>$icon {$task['title']}</li>\n";
}
echo "</ul>";
// Output:
// 
//   ✅ Buy groceries
//   ⬜ Write PHP lesson
//   ✅ Walk the dog
//   ⬜ Fix login bug
//

Modifying Values with Reference (&)

By default, foreach gives you a copy of each value. To modify the original array, use a reference (&):

<?php
$prices = [10.50, 20.00, 15.75, 8.99];

// This does NOT modify the original array (copy)
foreach ($prices as $price) {
    $price *= 1.10; // 10% increase — only modifies the copy
}
print_r($prices); // Still the original values!

// This DOES modify the original array (reference)
foreach ($prices as &$price) {
    $price *= 1.10; // 10% increase — modifies the original
}
unset($price); // ⚠️ Always unset the reference after the loop!

print_r($prices);
// Output: [11.55, 22.0, 17.325, 9.889]

⚠️ Always unset() After Reference Loops

After a foreach with &$value, the variable $value still points to the last array element. If you reuse that variable later, you'll accidentally modify the array:

$colors = ["red", "green", "blue"];
foreach ($colors as &$color) {
    $color = strtoupper($color);
}
// $color still references $colors[2]!

$color = "yellow";
print_r($colors); // ["RED", "GREEN", "yellow"] ← BUG!

// Fix: unset($color) right after the loop
foreach ($colors as &$color) {
    $color = strtoupper($color);
}
unset($color); // ✅ Breaks the reference

foreach with list() for Destructuring

<?php
$coordinates = [[1, 2], [3, 4], [5, 6]];

foreach ($coordinates as [$x, $y]) {
    echo "Point: ($x, $y)\n";
}
// Output:
// Point: (1, 2)
// Point: (3, 4)
// Point: (5, 6)

break and continue

These two keywords give you fine-grained control over loop execution.

Keyword	Effect	Analogy
`break`	Exits the loop entirely	Walking out of the building
`continue`	Skips the rest of the current iteration	Skipping one step on the staircase

break — Exit Early

<?php
// Find the first negative number
$numbers = [4, 7, 2, -3, 8, -1, 5];

foreach ($numbers as $num) {
    if ($num < 0) {
        echo "Found negative number: $num\n";
        break; // Stop searching — we found what we need
    }
    echo "Checked: $num (positive)\n";
}
// Output:
// Checked: 4 (positive)
// Checked: 7 (positive)
// Checked: 2 (positive)
// Found negative number: -3

continue — Skip an Iteration

<?php
// Print only even numbers
for ($i = 1; $i <= 10; $i++) {
    if ($i % 2 !== 0) {
        continue; // Skip odd numbers
    }
    echo "$i ";
}
// Output: 2 4 6 8 10

// Skip inactive users
$users = [
    ["name" => "Alice", "active" => true],
    ["name" => "Bob",   "active" => false],
    ["name" => "Carol", "active" => true],
    ["name" => "Dave",  "active" => false],
];

foreach ($users as $user) {
    if (!$user["active"]) {
        continue; // Skip inactive users
    }
    echo "Sending email to {$user['name']}...\n";
}
// Output:
// Sending email to Alice...
// Sending email to Carol...

break with Nested Loops

By default, break only exits the innermost loop. You can pass a number to break out of multiple levels:

<?php
// break 1 — exits inner loop only (default)
// break 2 — exits inner AND outer loop

for ($i = 0; $i < 3; $i++) {
    for ($j = 0; $j < 3; $j++) {
        if ($i === 1 && $j === 1) {
            break 2; // Exit BOTH loops
        }
        echo "[$i,$j] ";
    }
}
echo "\nDone!";
// Output: [0,0] [0,1] [0,2] [1,0]
// Done!

✅ When to Use break vs. continue

break: Use when you've found what you're looking for and don't need to keep looping (search results, error conditions, early exit).

continue: Use when certain items should be skipped but you still need to process the rest (filtering, skipping invalid data).

Nested Loops

A loop inside a loop. The inner loop runs completely for each iteration of the outer loop. This is essential for working with multi-dimensional data.

How Nested Loops Work

<?php
// Outer loop runs 3 times
// For each outer iteration, inner loop runs 4 times
// Total iterations: 3 × 4 = 12

for ($row = 1; $row <= 3; $row++) {
    for ($col = 1; $col <= 4; $col++) {
        echo "($row,$col) ";
    }
    echo "\n";
}
// Output:
// (1,1) (1,2) (1,3) (1,4)
// (2,1) (2,2) (2,3) (2,4)
// (3,1) (3,2) (3,3) (3,4)

Practical: Multiplication Table

<?php
echo "<table border='1'>\n";

// Header row
echo "<tr><th>×</th>";
for ($col = 1; $col <= 10; $col++) {
    echo "<th>$col</th>";
}
echo "</tr>\n";

// Data rows
for ($row = 1; $row <= 10; $row++) {
    echo "<tr><th>$row</th>";
    for ($col = 1; $col <= 10; $col++) {
        $product = $row * $col;
        echo "<td>$product</td>";
    }
    echo "</tr>\n";
}

echo "</table>";

Practical: Iterating Nested Arrays

<?php
$students = [
    ["name" => "Alice", "grades" => [92, 88, 95]],
    ["name" => "Bob",   "grades" => [78, 85, 72]],
    ["name" => "Carol", "grades" => [95, 97, 100]],
];

foreach ($students as $student) {
    $total = 0;
    $count = count($student["grades"]);

    foreach ($student["grades"] as $grade) {
        $total += $grade;
    }

    $average = $total / $count;
    echo "{$student['name']}: avg = " . number_format($average, 1) . "\n";
}
// Output:
// Alice: avg = 91.7
// Bob: avg = 78.3
// Carol: avg = 97.3

Practical: Building a Star Pattern

<?php
$rows = 5;

// Right triangle
for ($i = 1; $i <= $rows; $i++) {
    for ($j = 1; $j <= $i; $j++) {
        echo "⭐";
    }
    echo "\n";
}
// Output:
// ⭐
// ⭐⭐
// ⭐⭐⭐
// ⭐⭐⭐⭐
// ⭐⭐⭐⭐⭐

⚠️ Watch the Performance

Nested loops multiply their iterations. A loop of 1,000 inside a loop of 1,000 = 1,000,000 iterations. Three levels deep with 100 each = 1,000,000. Always be mindful of nested loop depth with large datasets.

Choosing the Right Loop

flowchart TD A["Need to repeat?"] --> B{"Iterating an array?"} B -->|Yes| C["foreach ✅"] B -->|No| D{"Know how many times?"} D -->|Yes| E["for"] D -->|No| F{"Must run at least once?"} F -->|Yes| G["do-while"] F -->|No| H["while"]

Loop	Best For	Example
`foreach`	Iterating arrays/objects	Display all products, process form data
`for`	Known number of iterations	Generate 10 rows, count from 1 to 100
`while`	Unknown iterations, check first	Read file lines, wait for condition
`do-while`	Must run at least once	Input validation, menu display, retry logic

Common Patterns

Pattern: Accumulator

<?php
// Sum all values in an array
$prices = [12.99, 24.50, 7.25, 18.00];
$total = 0;

foreach ($prices as $price) {
    $total += $price;
}

echo "Total: $" . number_format($total, 2);
// Output: Total: $62.74

Pattern: Search

<?php
// Find the first matching item
$users = ["alice", "bob", "carol", "dave"];
$search = "carol";
$found = false;

foreach ($users as $index => $user) {
    if ($user === $search) {
        echo "Found '$search' at index $index";
        $found = true;
        break;
    }
}

if (!$found) {
    echo "'$search' not found.";
}
// Output: Found 'carol' at index 2

Pattern: Filter

<?php
// Build a new array with only items that pass a test
$numbers = [3, 7, 12, 5, 18, 2, 9, 15];
$bigNumbers = [];

foreach ($numbers as $num) {
    if ($num >= 10) {
        $bigNumbers[] = $num;
    }
}

print_r($bigNumbers);
// Output: [12, 18, 15]

Pattern: Transform

<?php
// Create a new array by transforming each element
$names = ["alice", "bob", "carol"];
$formatted = [];

foreach ($names as $name) {
    $formatted[] = ucfirst($name);
}

print_r($formatted);
// Output: ["Alice", "Bob", "Carol"]

💡 Preview: In Lesson 9, you'll learn array_filter(), array_map(), and array_reduce() — built-in functions that replace many of these manual loop patterns with one-liners.

Hands-On Exercises

🏋️ Exercise 1: FizzBuzz

Objective: The classic programming challenge! Print numbers 1 to 30, but:

For multiples of 3, print "Fizz" instead of the number
For multiples of 5, print "Buzz" instead of the number
For multiples of both 3 and 5, print "FizzBuzz"

💡 Hint

Check for divisibility by both 3 AND 5 first (otherwise the individual checks will match first). Use the modulo operator (%) — a number is divisible by N if $num % N === 0.

✅ Solution

<?php
for ($i = 1; $i <= 30; $i++) {
    if ($i % 3 === 0 && $i % 5 === 0) {
        echo "FizzBuzz";
    } elseif ($i % 3 === 0) {
        echo "Fizz";
    } elseif ($i % 5 === 0) {
        echo "Buzz";
    } else {
        echo $i;
    }
    echo "\n";
}
// 1, 2, Fizz, 4, Buzz, Fizz, 7, 8, Fizz, Buzz,
// 11, Fizz, 13, 14, FizzBuzz, 16, ...

🏋️ Exercise 2: Shopping Cart Total

Objective: Use foreach to calculate the total cost of a shopping cart, applying discounts and formatting the output.

Starter Code:

<?php
$cart = [
    ["name" => "Widget",     "price" => 9.99,  "qty" => 3],
    ["name" => "Gadget",     "price" => 24.99, "qty" => 1],
    ["name" => "Doohickey",  "price" => 4.50,  "qty" => 5],
    ["name" => "Thingamajig","price" => 15.00, "qty" => 2],
];

$discountThreshold = 50; // Orders over $50 get 10% off

// TODO: Loop through cart, calculate line totals and grand total
// TODO: Apply discount if over threshold
// TODO: Display formatted receipt

💡 Hint

Use an accumulator variable for the grand total. For each item, calculate $price * $qty. After the loop, check if the total exceeds the threshold and apply a 10% discount.

✅ Solution

<?php
$cart = [
    ["name" => "Widget",     "price" => 9.99,  "qty" => 3],
    ["name" => "Gadget",     "price" => 24.99, "qty" => 1],
    ["name" => "Doohickey",  "price" => 4.50,  "qty" => 5],
    ["name" => "Thingamajig","price" => 15.00, "qty" => 2],
];

$discountThreshold = 50;
$subtotal = 0;

echo "===== SHOPPING CART =====\n";
echo str_pad("Item", 15) . str_pad("Qty", 5) . str_pad("Price", 10) . "Line Total\n";
echo str_repeat("-", 45) . "\n";

foreach ($cart as $item) {
    $lineTotal = $item["price"] * $item["qty"];
    $subtotal += $lineTotal;

    echo str_pad($item["name"], 15);
    echo str_pad($item["qty"], 5);
    echo str_pad("$" . number_format($item["price"], 2), 10);
    echo "$" . number_format($lineTotal, 2) . "\n";
}

echo str_repeat("-", 45) . "\n";
echo "Subtotal: $" . number_format($subtotal, 2) . "\n";

if ($subtotal > $discountThreshold) {
    $discount = $subtotal * 0.10;
    $total = $subtotal - $discount;
    echo "Discount (10%): -$" . number_format($discount, 2) . "\n";
    echo "TOTAL: $" . number_format($total, 2) . "\n";
} else {
    echo "TOTAL: $" . number_format($subtotal, 2) . "\n";
    echo "(Spend over $$discountThreshold for 10% off!)\n";
}

🏋️ Exercise 3: Password Validator

Objective: Use a for loop (and optionally while) to check password strength by iterating through each character.

Rules:

At least 8 characters long
Contains at least one uppercase letter
Contains at least one lowercase letter
Contains at least one digit
Contains at least one special character (!@#$%^&*)

💡 Hint

Use ctype_upper(), ctype_lower(), ctype_digit() to check individual characters. Use strpos() or str_contains() with a string of special characters.

✅ Solution

<?php
$password = "MyP@ss1234";

$hasUpper = false;
$hasLower = false;
$hasDigit = false;
$hasSpecial = false;
$specials = "!@#$%^&*";
$errors = [];

// Check length first
if (strlen($password) < 8) {
    $errors[] = "Must be at least 8 characters";
}

// Check each character
for ($i = 0; $i < strlen($password); $i++) {
    $char = $password[$i];

    if (ctype_upper($char)) {
        $hasUpper = true;
    } elseif (ctype_lower($char)) {
        $hasLower = true;
    } elseif (ctype_digit($char)) {
        $hasDigit = true;
    } elseif (str_contains($specials, $char)) {
        $hasSpecial = true;
    }
}

if (!$hasUpper)   $errors[] = "Missing uppercase letter";
if (!$hasLower)   $errors[] = "Missing lowercase letter";
if (!$hasDigit)   $errors[] = "Missing digit";
if (!$hasSpecial) $errors[] = "Missing special character";

if (empty($errors)) {
    echo "✅ Password is strong!\n";
} else {
    echo "❌ Password is weak:\n";
    foreach ($errors as $error) {
        echo "  - $error\n";
    }
}
// Output: ✅ Password is strong!

🎯 Quick Quiz

Question 1: Which loop is best for iterating over an associative array?

Question 2: What does continue do inside a loop?

Question 3: What is the key difference between while and do-while?

Question 4: Why should you call unset($value) after a foreach loop that uses &$value?

Question 5: What does break 2 do inside nested loops?

Summary

🎉 Key Takeaways

for — Best for counted iterations; packs init, condition, and update in one line
while — Best when you don't know the iteration count; checks condition before each pass
do-while — Like while, but guarantees at least one execution; great for validation and retries
foreach — PHP's go-to for arrays; use $key => $value for associative arrays
break — Exits the loop entirely; use break N to exit N levels of nesting
continue — Skips the rest of the current iteration and moves to the next
References — Use &$value in foreach to modify the original array; always unset() after
Performance — Watch nested loop depth; avoid function calls in loop conditions

📚 Additional Resources

🚀 What's Next?

Now that you can repeat code and iterate data, it's time to organize your code. In Lesson 7: Functions, you'll learn to bundle reusable logic into functions — with parameters, return values, type hints, and more.

🎉 Congratulations!

You've mastered PHP's loop toolkit! From for to foreach, you can now iterate, search, filter, and transform data like a pro. Module 2 is almost complete!

📑 In This Lesson

Why Loops?

📖 Definition

The for Loop

Syntax

Basic Examples

Practical: Building an HTML Table

Looping Through Strings

⚠️ Performance Tip

Multiple Expressions

The while Loop

Syntax

Basic Examples

Practical: Reading User Input (Simulated)

Practical: Halving Until Small Enough

⚠️ Infinite Loops!

The do-while Loop

Syntax

while vs. do-while

Practical: Input Validation Pattern

Practical: Generating Unique IDs

✅ When to Use do-while

The foreach Loop

Syntax: Values Only

Syntax: Keys and Values

Indexed Arrays

Associative Arrays

Practical: Rendering an HTML List

Modifying Values with Reference (&)

⚠️ Always unset() After Reference Loops

foreach with list() for Destructuring

break and continue

break — Exit Early

continue — Skip an Iteration

break with Nested Loops

✅ When to Use break vs. continue

Nested Loops

How Nested Loops Work

Practical: Multiplication Table

Practical: Iterating Nested Arrays

Practical: Building a Star Pattern

⚠️ Watch the Performance

Choosing the Right Loop

Common Patterns

Pattern: Accumulator

Pattern: Search

Pattern: Filter

Pattern: Transform

Hands-On Exercises

🏋️ Exercise 1: FizzBuzz

🏋️ Exercise 2: Shopping Cart Total

Starter Code:

🏋️ Exercise 3: Password Validator

Rules:

🎯 Quick Quiz

Summary

🎉 Key Takeaways

📚 Additional Resources

🚀 What's Next?

🎉 Congratulations!