PHP Control Flow — The Complete Guide with Real-World Examples

<?php
// io.thecodeforge: If/Elseif/Else — Grade Evaluator
function evaluateGrade($score) {
    echo "Score: $score — ";
    if ($score >= 90) {
        echo "Distinction: Excellent work!" . PHP_EOL;
    } elseif ($score >= 75) {
        echo "First Class: Very good!" . PHP_EOL;
    } elseif ($score >= 60) {
        echo "Second Class: Good, but keep improving." . PHP_EOL;
    } elseif ($score >= 35) {
        echo "Pass: Just passed. Work harder next time." . PHP_EOL;
    } else {
        echo "Fail: Needs improvement." . PHP_EOL;
    }
}

evaluateGrade(95);  // Distinction
evaluateGrade(82);  // First Class
evaluateGrade(67);  // Second Class
evaluateGrade(42);  // Pass
evaluateGrade(28);  // Fail

// ORDER MATTERS — WRONG EXAMPLE
function wrongGradeOrder($score) {
    echo "Score: $score (WRONG ORDER) — ";
    if ($score >= 60) {
        echo "Pass" . PHP_EOL;      // This catches everything >= 60
    } elseif ($score >= 90) {
        echo "Distinction" . PHP_EOL; // NEVER runs!
    }
}
wrongGradeOrder(95); // Outputs 'Pass' instead of 'Distinction'

<?php
// io.thecodeforge: Switch Statement with Intentional vs Accidental Fall-through

function checkDeliveryStatus($statusCode) {
    echo "Status $statusCode: ";
    
    switch ($statusCode) {
        case 1:
            echo "Order placed — ";
            // Intentional fall-through: case 1 and 2 both proceed to 'processing'
        case 2:
            echo "Processing — ";
            // Intentional fall-through into case 3
        case 3:
            echo "Shipped";
            break;
            
        case 4:
            echo "Out for delivery";
            break;
            
        case 5:
            echo "Delivered";
            break;
            
        default:
            echo "Unknown status";
    }
    echo PHP_EOL;
}

checkDeliveryStatus(1); // Order placed — Processing — Shipped
checkDeliveryStatus(2); // Processing — Shipped
checkDeliveryStatus(3); // Shipped
checkDeliveryStatus(4); // Out for delivery
checkDeliveryStatus(5); // Delivered
checkDeliveryStatus(99); // Unknown status

// ACCIDENTAL FALL-THROUGH (the payment processor bug)
function paymentProcessor($status) {
    echo "Payment $status: ";
    switch ($status) {
        case 'pending':
            echo "Processing payment — ";
            // MISSING BREAK — disaster!
        case 'completed':
            echo "Sending confirmation — ";
            // MISSING BREAK
        case 'failed':
            echo "Logging failure.";
            break;
    }
    echo PHP_EOL;
}

paymentProcessor('pending');   // Processing payment — Sending confirmation — Logging failure.
paymentProcessor('completed'); // Sending confirmation — Logging failure.
paymentProcessor('failed');    // Logging failure.

<?php
// io.thecodeforge: The Four Loop Types

echo "=== FOR: Multiplication Table ===" . PHP_EOL;
$number = 5;
for ($i = 1; $i <= 10; $i++) {
    echo "$number x $i = " . ($number * $i) . PHP_EOL;
}

echo PHP_EOL . "=== WHILE: Countdown Until Condition Met ===" . PHP_EOL;
$countdown = 5;
while ($countdown > 0) {
    echo "T-minus $countdown seconds..." . PHP_EOL;
    $countdown--;
}
echo "Liftoff!" . PHP_EOL;

echo PHP_EOL . "=== DO-WHILE: Runs at Least Once ===" . PHP_EOL;
$attempts = 0;
do {
    $attempts++;
    echo "Attempt $attempts: Connecting to service..." . PHP_EOL;
} while ($attempts < 3 && rand(0, 1) === 0); // Simulates random success
echo "Connection established after $attempts attempt(s)." . PHP_EOL;

echo PHP_EOL . "=== FOREACH: Processing Collection ===" . PHP_EOL;
$items = ['Apple', 'Banana', 'Cherry', 'Date'];
foreach ($items as $index => $item) {
    echo "Item $index: $item" . PHP_EOL;
}

echo PHP_EOL . "=== break vs continue ===" . PHP_EOL;
$numbers = range(1, 10);

echo "Using continue (skip even numbers): ";
foreach ($numbers as $num) {
    if ($num % 2 == 0) {
        continue; // skip this iteration
    }
    echo "$num ";
}
echo PHP_EOL;

echo "Using break (stop at 5): ";
foreach ($numbers as $num) {
    if ($num > 5) {
        break; // exit loop entirely
    }
    echo "$num ";
}
echo PHP_EOL;

<?php
// io.thecodeforge: PHP Type Juggling in Conditionals
// Shows the difference between == (loose) and === (strict)

echo "=== Loose Comparison (==) Surprises ===" . PHP_EOL;

// These all evaluate to TRUE with == — most are surprising
var_dump(0 == '0');          // true  — int 0 equals string '0'
var_dump(0 == '');           // true  — int 0 equals empty string
var_dump(0 == false);        // true  — int 0 equals boolean false
var_dump('0' == false);      // true  — string '0' equals boolean false
var_dump('' == false);       // true  — empty string equals boolean false
var_dump(null == false);     // true  — null equals boolean false
var_dump('false' == false);  // FALSE — 'false' is a non-empty string (truthy!)

echo PHP_EOL . "=== Strict Comparison (===) — Always Safe ===" . PHP_EOL;

var_dump(0 === '0');         // false — int is not string
var_dump(0 === '');          // false — int is not string
var_dump(0 === false);       // false — int is not bool
var_dump('0' === false);     // false — string is not bool
var_dump(null === false);    // false — null is not bool

echo PHP_EOL . "=== isset() vs empty() ===" . PHP_EOL;

$undefinedVariable = null;

// isset: does the variable exist and is not null?
var_dump(isset($undefinedVariable));  // false — it's null

// empty: is the value falsy? (includes '', 0, null, false, [], '0')
var_dump(empty($undefinedVariable));  // true — null is empty
var_dump(empty(0));                   // true — 0 is empty
var_dump(empty('0'));                 // true — '0' is empty
var_dump(empty('hello'));             // false — non-empty string is not empty

echo PHP_EOL . "=== Safe Pattern: Always Use ===" . PHP_EOL;

$loginInput = '0';  // User submitted the string '0'

// DANGEROUS: loose comparison
if ($loginInput == true) {
    echo "Loose: '$loginInput' is truthy (WRONG for login checks!)" . PHP_EOL;
}

// SAFE: strict comparison
if ($loginInput === true) {
    echo "Strict: '$loginInput' is truthy" . PHP_EOL;
} else {
    echo "Strict: '$loginInput' is NOT boolean true (correct!)" . PHP_EOL;
}

<?php
// io.thecodeforge: Switch Statement with Intentional vs Accidental Fall-through

function checkDeliveryStatus($statusCode) {
    echo "Status $statusCode: ";
    
    switch ($statusCode) {
        case 1:
            echo "Order placed — ";
            // Intentional fall-through: case 1 and 2 both proceed to 'processing'
        case 2:
            echo "Processing — ";
            // Intentional fall-through into case 3
        case 3:
            echo "Shipped";
            break;
            
        case 4:
            echo "Out for delivery";
            break;
            
        case 5:
            echo "Delivered";
            break;
            
        default:
            echo "Unknown status";
    }
    echo PHP_EOL;
}

checkDeliveryStatus(1); // Order placed — Processing — Shipped
checkDeliveryStatus(2); // Processing — Shipped
checkDeliveryStatus(3); // Shipped
checkDeliveryStatus(4); // Out for delivery
checkDeliveryStatus(5); // Delivered
checkDeliveryStatus(99); // Unknown status

// ACCIDENTAL FALL-THROUGH (the payment processor bug)
function paymentProcessor($status) {
    echo "Payment $status: ";
    switch ($status) {
        case 'pending':
            echo "Processing payment — ";
            // MISSING BREAK — disaster!
        case 'completed':
            echo "Sending confirmation — ";
            // MISSING BREAK
        case 'failed':
            echo "Logging failure.";
            break;
    }
    echo PHP_EOL;
}

paymentProcessor('pending');   // Processing payment — Sending confirmation — Logging failure.
paymentProcessor('completed'); // Sending confirmation — Logging failure.
paymentProcessor('failed');    // Logging failure.

<?php
// io.thecodeforge: Nested Control Flow Examples

echo "=== Shopping Cart Validation (Nested Loop + Conditions) ===" . PHP_EOL;

$cart = [
    ['name' => 'Widget A', 'price' => 25.00, 'quantity' => 3],
    ['name' => 'Widget B', 'price' => 0,    'quantity' => 1],
    ['name' => 'Widget C', 'price' => 49.99, 'quantity' => 0],
    ['name' => 'Widget D', 'price' => 99.99, 'quantity' => 2],
];

$validItems = [];
$totalPrice = 0;

foreach ($cart as $item) {
    // Skip items with zero or negative quantity
    if ($item['quantity'] <= 0) {
        echo "Skipped: {$item['name']} (quantity is {$item['quantity']})" . PHP_EOL;
        continue;
    }
    // Skip items with zero price (likely a data error)
    if ($item['price'] <= 0) {
        echo "Skipped: {$item['name']} (price is {$item['price']})" . PHP_EOL;
        continue;
    }
    // Item is valid — add to cart total
    $lineTotal = $item['price'] * $item['quantity'];
    $validItems[] = $item;
    $totalPrice += $lineTotal;
    echo "Added: {$item['name']} x{$item['quantity']} = $" . number_format($lineTotal, 2) . PHP_EOL;
}
echo "Cart total: $" . number_format($totalPrice, 2) . PHP_EOL;

echo PHP_EOL;

// THE FOREACH REFERENCE TRAP
echo "=== Foreach Reference Trap ===" . PHP_EOL;

$colors = ['red', 'green', 'blue'];

// First loop: modify values using reference (&)
foreach ($colors as &$color) {
    $color = strtoupper($color);
}
// BUG: $color is STILL a reference to the LAST element ($colors[2])
// At this point: $colors = ['RED', 'GREEN', 'BLUE']
// But $color points to $colors[2] which is 'BLUE'

// Second loop: reusing $color without realizing it's still a reference
foreach ($colors as $color) {
    // Each iteration assigns to $color, which still points to $colors[2]
    // So $colors[2] gets overwritten on EVERY iteration
}

echo "After trap: ";
var_dump($colors);
// Expected: ['RED', 'GREEN', 'BLUE']
// Actual:   ['RED', 'GREEN', 'RED']  — last element is corrupted!

// FIX: unset the reference after the first loop
$colors2 = ['red', 'green', 'blue'];
foreach ($colors2 as &$color2) {
    $color2 = strtoupper($color2);
}
unset($color2); // CRITICAL: breaks the reference

foreach ($colors2 as $color2) {
    // Safe now — $color2 is a normal variable, not a reference
}

echo "After fix:  ";
var_dump($colors2);
// Correct: ['RED', 'GREEN', 'BLUE']

<?php
// io.thecodeforge: Ternary, Null Coalescing, and Match

echo "=== Ternary Operator (if/else shortcut) ===" . PHP_EOL;
$cartCount = 3;
$message = $cartCount > 0 ? "You have $cartCount items in your cart." : "Your cart is empty.";
echo $message . PHP_EOL;

$cartCount = 0;
$message = $cartCount > 0 ? "You have $cartCount items in your cart." : "Your cart is empty.";
echo $message . PHP_EOL;

echo PHP_EOL . "=== Null Coalescing Operator (??) — Safe Defaults ===" . PHP_EOL;
// Without null coalescing
$name = isset($_GET['name']) ? $_GET['name'] : 'Guest';
echo "Hello, $name!" . PHP_EOL;

// With null coalescing (much cleaner)
$name = $_GET['name'] ?? 'Guest';
echo "Hello, $name!" . PHP_EOL;

// Chaining multiple fallbacks
$config = ['theme' => null, 'language' => 'en'];
$theme = $config['theme'] ?? $config['default_theme'] ?? 'light';
echo "Theme: $theme" . PHP_EOL;

echo PHP_EOL . "=== MATCH EXPRESSION (PHP 8.0+) ===" . PHP_EOL;
$httpStatusCode = 404;

$statusDescription = match($httpStatusCode) {
    200 => 'OK — Request succeeded.',
    201 => 'Created — Resource successfully created.',
    301 => 'Moved Permanently — Redirect to new URL.',
    400 => 'Bad Request — Check your request parameters.',
    401 => 'Unauthorized — Authentication required.',
    403 => 'Forbidden — You do not have permission.',
    404 => 'Not Found — This resource does not exist.',
    500 => 'Internal Server Error — Something went wrong.',
    default => 'Unknown status code.'
};
echo "HTTP $httpStatusCode: $statusDescription" . PHP_EOL;

// Match with multiple values per arm
$currentMonth = 8;
$season = match(true) {
    in_array($currentMonth, [12, 1, 2])  => 'Winter',
    in_array($currentMonth, [3, 4, 5])   => 'Spring',
    in_array($currentMonth, [6, 7, 8])   => 'Summer',
    in_array($currentMonth, [9, 10, 11]) => 'Autumn',
    default => 'Unknown'
};
echo "Month $currentMonth is in $season." . PHP_EOL;