🔀 Lesson 5: Control Flow

What Is Control Flow?

By default, PHP executes code from top to bottom, line by line. Control flow structures change this — they create branches where different code runs depending on whether a condition is true or false.

PHP gives you three main tools for branching:

if / elseif / else

The if statement is the most fundamental control structure in any programming language. It says: "If this condition is true, run this code."

Basic if

<?php
$temperature = 35;

if ($temperature > 30) {
    echo "It's hot outside! Stay hydrated. 🥵";
}
// This only runs if the condition is true

if / else

Add else to handle the case when the condition is not true:

<?php
$hour = 14;

if ($hour < 12) {
    echo "Good morning! ☀️";
} else {
    echo "Good afternoon! 🌤️";
}

if / elseif / else

Chain multiple conditions together with elseif:

<?php
$score = 85;

if ($score >= 90) {
    $grade = "A";
    $message = "Excellent work!";
} elseif ($score >= 80) {
    $grade = "B";
    $message = "Good job!";
} elseif ($score >= 70) {
    $grade = "C";
    $message = "Not bad, keep it up!";
} elseif ($score >= 60) {
    $grade = "D";
    $message = "You passed, but let's improve.";
} else {
    $grade = "F";
    $message = "Let's review the material together.";
}

echo "Grade: $grade — $message";
// Output: Grade: B — Good job!

// BUG: This always prints "Passing" for any score >= 60
$score = 95;
if ($score >= 60) {
    echo "Passing";     // This matches first!
} elseif ($score >= 90) {
    echo "Excellent";   // Never reached for 95!
}

// FIX: Check the most specific condition first
if ($score >= 90) {
    echo "Excellent";   // Checks this first ✓
} elseif ($score >= 60) {
    echo "Passing";
}

Single-Line Conditions

For simple one-line blocks, you can omit the curly braces — but it's not recommended:

<?php
// This works, but DON'T do it:
if ($loggedIn) echo "Welcome back!";

// This is safer and clearer:
if ($loggedIn) {
    echo "Welcome back!";
}

// Why? Because this common mistake is a silent bug:
if ($loggedIn)
    echo "Welcome back!";
    echo "Loading dashboard..."; // ← ALWAYS runs! Not inside the if!

Real-World Example: User Authentication

<?php
$username = "admin";
$password = "s3cur3P@ss";
$isLocked = false;
$loginAttempts = 2;

if ($isLocked) {
    echo "Account is locked. Contact support.";
} elseif ($loginAttempts >= 5) {
    echo "Too many failed attempts. Account locked.";
    $isLocked = true;
} elseif ($username === "admin" && $password === "s3cur3P@ss") {
    echo "Login successful! Welcome, $username.";
    $loginAttempts = 0;
} else {
    $loginAttempts++;
    echo "Invalid credentials. Attempt $loginAttempts of 5.";
}

Truthy and Falsy Values

When PHP evaluates a condition, it doesn't require a boolean. It can evaluate any value as either truthy or falsy. Understanding this is crucial for writing clean, idiomatic PHP.

The Complete Falsy List

PHP has exactly eight falsy values. Everything else is truthy.

<?php
// Truthy examples — all of these run the if block
if (1)           { echo "truthy\n"; }  // Non-zero integer
if (-1)          { echo "truthy\n"; }  // Negative numbers are truthy!
if (0.5)         { echo "truthy\n"; }  // Non-zero float
if ("hello")     { echo "truthy\n"; }  // Non-empty string
if ("false")     { echo "truthy\n"; }  // ⚠️ The STRING "false" is truthy!
if (" ")         { echo "truthy\n"; }  // Space is a non-empty string
if ([0])         { echo "truthy\n"; }  // Non-empty array

// Falsy examples — none of these run the if block
if (false) { echo "won't run\n"; }
if (0)     { echo "won't run\n"; }
if (0.0)   { echo "won't run\n"; }
if ("")    { echo "won't run\n"; }
if ("0")   { echo "won't run\n"; }  // ⚠️ Watch out!
if ([])    { echo "won't run\n"; }
if (null)  { echo "won't run\n"; }

Using Truthy/Falsy in Practice

<?php
// Checking if a string has content
$name = "Alice";
if ($name) {
    echo "Hello, $name!";   // Runs — $name is truthy
}

// Checking if an array has items
$cart = [];
if ($cart) {
    echo "Your cart has items.";
} else {
    echo "Your cart is empty."; // Runs — empty array is falsy
}

// Checking if a variable exists and has a value
$email = $_POST['email'] ?? null;
if ($email) {
    echo "Processing email: $email";
} else {
    echo "No email provided.";
}

// BAD — rejects valid quantity of 0
$quantity = 0;
if ($quantity) { /* never runs for 0! */ }

// GOOD — explicitly check what you mean
if ($quantity !== null) { /* runs for 0 */ }
if (isset($quantity))   { /* runs for 0 */ }

Nested Conditionals

You can place if statements inside other if statements to handle complex logic. But be careful — deep nesting makes code hard to read.

Basic Nesting

<?php
$age = 25;
$hasLicense = true;
$hasCar = false;

if ($age >= 16) {
    if ($hasLicense) {
        if ($hasCar) {
            echo "You can drive your own car!";
        } else {
            echo "You have a license but need a car.";
        }
    } else {
        echo "You're old enough, but need a license first.";
    }
} else {
    echo "You're too young to drive.";
}

The Problem with Deep Nesting

Every level of nesting makes code harder to follow. The example above is already getting difficult — imagine adding more conditions!

Flattening with Early Returns / Guard Clauses

A guard clause checks for a disqualifying condition and exits early. This eliminates nesting:

<?php
function canDrive($age, $hasLicense, $hasCar): string
{
    // Guard clauses — check disqualifiers first
    if ($age < 16) {
        return "You're too young to drive.";
    }

    if (!$hasLicense) {
        return "You're old enough, but need a license first.";
    }

    if (!$hasCar) {
        return "You have a license but need a car.";
    }

    // If we reach here, all conditions are met
    return "You can drive your own car!";
}

echo canDrive(25, true, false);
// Output: You have a license but need a car.

Combining Conditions Instead of Nesting

Sometimes you can flatten nested if statements by combining conditions with &&:

<?php
// Nested (harder to read)
if ($isLoggedIn) {
    if ($isAdmin) {
        if ($hasPermission) {
            deleteUser($userId);
        }
    }
}

// Flat (easier to read, same logic)
if ($isLoggedIn && $isAdmin && $hasPermission) {
    deleteUser($userId);
}

The switch Statement

When you're comparing one value against many possible matches, a switch statement is cleaner than a long if/elseif chain.

Basic Syntax

<?php
$day = "Wednesday";

switch ($day) {
    case "Monday":
        echo "Start of the work week 😩";
        break;
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
        echo "Midweek grind 💪";
        break;
    case "Friday":
        echo "Almost the weekend! 🎉";
        break;
    case "Saturday":
    case "Sunday":
        echo "Weekend! 🏖️";
        break;
    default:
        echo "That's not a valid day.";
        break;
}
// Output: Midweek grind 💪

How switch Works

$fruit = "apple";

switch ($fruit) {
    case "apple":
        echo "Apple\n";     // Runs
        // Missing break!
    case "banana":
        echo "Banana\n";    // ALSO runs! (fall-through)
        break;
    case "cherry":
        echo "Cherry\n";    // Doesn't run (break stopped it)
        break;
}
// Output:
// Apple
// Banana

Intentional Fall-Through

Sometimes fall-through is useful — for grouping multiple cases that share the same code:

<?php
$month = 3; // March

switch ($month) {
    case 12:
    case 1:
    case 2:
        $season = "Winter";
        break;
    case 3:
    case 4:
    case 5:
        $season = "Spring";
        break;
    case 6:
    case 7:
    case 8:
        $season = "Summer";
        break;
    case 9:
    case 10:
    case 11:
        $season = "Fall";
        break;
    default:
        $season = "Unknown";
}

echo "$season"; // Spring

switch Uses Loose Comparison!

A critical detail: switch uses == (loose comparison), not ===. This means type juggling can surprise you:

<?php
$value = 0;

switch ($value) {
    case "hello":
        echo "Matched 'hello'!"; // ⚠️ This runs! 0 == "hello" is true
        break;
    case 0:
        echo "Matched 0";
        break;
}
// Output: Matched 'hello'!
// Because 0 == "hello" → true (string becomes 0 via type juggling)

💡 Tip: If you need strict comparison, use PHP 8's match expression instead — it uses === by default.

Returning from switch

When using switch inside a function, return exits both the case and the function — no break needed:

<?php
function getStatusMessage(string $status): string
{
    switch ($status) {
        case "pending":
            return "Your order is being processed.";
        case "shipped":
            return "Your order is on its way!";
        case "delivered":
            return "Your order has arrived.";
        case "cancelled":
            return "Your order was cancelled.";
        default:
            return "Unknown status: $status";
    }
}

echo getStatusMessage("shipped");
// Output: Your order is on its way!

PHP 8's match Expression

PHP 8 introduced match — a modern alternative to switch that fixes most of its footguns. It's one of the best additions to the language.

Basic Syntax

<?php
$statusCode = 404;

$message = match ($statusCode) {
    200 => "OK",
    301 => "Moved Permanently",
    404 => "Not Found",
    500 => "Internal Server Error",
    default => "Unknown Status",
};

echo $message; // "Not Found"

match vs. switch — Key Differences

Strict Comparison (No Type Juggling)

<?php
$value = 0;

// switch would match "hello" (0 == "hello" is true)
// match does NOT — it uses ===
$result = match ($value) {
    "hello" => "Matched hello",
    0       => "Matched zero",   // ✅ This matches (0 === 0)
    default => "No match",
};

echo $result; // "Matched zero"

Multiple Values Per Arm

<?php
$month = 3;

$season = match (true) {
    in_array($month, [12, 1, 2])  => "Winter",
    in_array($month, [3, 4, 5])   => "Spring",
    in_array($month, [6, 7, 8])   => "Summer",
    in_array($month, [9, 10, 11]) => "Fall",
    default                        => "Unknown",
};

echo $season; // "Spring"

// Or more simply, using comma-separated values:
$season = match ($month) {
    12, 1, 2  => "Winter",
    3, 4, 5   => "Spring",
    6, 7, 8   => "Summer",
    9, 10, 11 => "Fall",
    default   => "Unknown",
};

echo $season; // "Spring"

match with Complex Conditions

By matching against true, you can use match like a supercharged if/elseif chain:

<?php
$score = 85;

$grade = match (true) {
    $score >= 90 => "A",
    $score >= 80 => "B",
    $score >= 70 => "C",
    $score >= 60 => "D",
    default      => "F",
};

echo "Grade: $grade"; // "Grade: B"

No match = Error

Unlike switch (which silently does nothing), match throws an error if no arm matches and there's no default:

<?php
$color = "purple";

// This throws UnhandledMatchError!
$hex = match ($color) {
    "red"   => "#FF0000",
    "green" => "#00FF00",
    "blue"  => "#0000FF",
    // No default — "purple" causes an error
};

Real-World Example: HTTP Response Handler

<?php
function handleResponse(int $code): string
{
    return match (true) {
        $code >= 200 && $code < 300 => "✅ Success",
        $code >= 300 && $code < 400 => "↪️ Redirect",
        $code === 401                  => "🔒 Unauthorized — please log in",
        $code === 403                  => "🚫 Forbidden — access denied",
        $code === 404                  => "❓ Not Found",
        $code === 429                  => "⏳ Too Many Requests — slow down",
        $code >= 500                   => "💥 Server Error — try again later",
        default                        => "❔ Unknown status code: $code",
    };
}

echo handleResponse(200);  // ✅ Success
echo handleResponse(404);  // ❓ Not Found
echo handleResponse(503);  // 💥 Server Error — try again later

Choosing the Right Tool

Here's a quick decision guide for which control flow structure to use:

Quick Reference

Alternative Syntax for Templates

When mixing PHP with HTML, PHP offers an alternative colon-based syntax that's easier to read in templates:

<?php $role = "admin"; ?>

<!-- Alternative if/elseif/else syntax -->
<?php if ($role === "admin"): ?>
    <div class="admin-panel">
        <h2>Admin Dashboard</h2>
    </div>
<?php elseif ($role === "editor"): ?>
    <div class="editor-panel">
        <h2>Editor Dashboard</h2>
    </div>
<?php else: ?>
    <div class="user-panel">
        <h2>Welcome</h2>
    </div>
<?php endif; ?>

<!-- Alternative switch syntax -->
<?php switch ($role):
    case "admin": ?>
        <span class="badge-admin">Admin</span>
        <?php break; ?>
    <?php case "editor": ?>
        <span class="badge-editor">Editor</span>
        <?php break; ?>
    <?php default: ?>
        <span class="badge-user">User</span>
<?php endswitch; ?>

💡 Note: The colon syntax (if(): ... endif;) is mainly used when writing HTML templates with embedded PHP. In pure PHP files, always use the standard curly-brace syntax.

Hands-On Exercises

<?php
$age = 30;
$isMember = true;

// TODO: Calculate $price based on age
// TODO: Apply 25% member discount if applicable
// TODO: Output the final price

<?php
$age = 30;
$isMember = true;

// Determine base price by age
if ($age < 5) {
    $price = 0;
} elseif ($age <= 12) {
    $price = 8;
} elseif ($age <= 17) {
    $price = 12;
} elseif ($age <= 64) {
    $price = 20;
} else {
    $price = 10;
}

// Apply member discount
$discount = 0;
if ($isMember && $price > 0) {
    $discount = $price * 0.25;
    $price -= $discount;
}

echo "Age: $age\n";
echo "Member: " . ($isMember ? "Yes" : "No") . "\n";
if ($discount > 0) {
    echo "Discount: -$" . number_format($discount, 2) . "\n";
}
echo "Price: $" . number_format($price, 2) . "\n";
// Output:
// Age: 30
// Member: Yes
// Discount: -$5.00
// Price: $15.00

<?php
$today = date("l"); // e.g., "Monday"

$activity = match ($today) {
    "Monday"    => "Team standup & sprint planning",
    "Tuesday"   => "Deep work — no meetings",
    "Wednesday" => "Code review & pair programming",
    "Thursday"  => "Feature development",
    "Friday"    => "Deploy & retrospective",
    "Saturday", "Sunday" => "Rest & personal projects",
};

$workout = match ($today) {
    "Monday", "Thursday"         => "🏋️ Strength training",
    "Tuesday", "Friday"          => "🏃 Cardio",
    "Wednesday"                  => "🧘 Yoga",
    "Saturday", "Sunday"         => "🚶 Walk or rest",
};

$emoji = match ($today) {
    "Monday"    => "🚀",
    "Tuesday"   => "🎯",
    "Wednesday" => "👥",
    "Thursday"  => "💻",
    "Friday"    => "🎉",
    "Saturday", "Sunday" => "😎",
};

echo "===== $emoji $today =====\n";
echo "Activity: $activity\n";
echo "Workout:  $workout\n";

<?php
$score = 85;

// === Approach 1: if/elseif ===
if ($score >= 90) {
    $grade1 = "A";
} elseif ($score >= 80) {
    $grade1 = "B";
} elseif ($score >= 70) {
    $grade1 = "C";
} elseif ($score >= 60) {
    $grade1 = "D";
} else {
    $grade1 = "F";
}

// === Approach 2: switch (with intdiv trick) ===
switch (intdiv($score, 10)) {
    case 10:
    case 9:
        $grade2 = "A";
        break;
    case 8:
        $grade2 = "B";
        break;
    case 7:
        $grade2 = "C";
        break;
    case 6:
        $grade2 = "D";
        break;
    default:
        $grade2 = "F";
}

// === Approach 3: match (cleanest!) ===
$grade3 = match (true) {
    $score >= 90 => "A",
    $score >= 80 => "B",
    $score >= 70 => "C",
    $score >= 60 => "D",
    default      => "F",
};

echo "if/elseif: $grade1\n";  // B
echo "switch:    $grade2\n";  // B
echo "match:     $grade3\n";  // B

Summary

📚 Additional Resources

• PHP Manual: if
• PHP Manual: switch
• PHP Manual: match (PHP 8)
• PHP Manual: Type Comparison Tables

🚀 What's Next?

Now that your code can make decisions, it's time to make it repeat things. In Lesson 6: Loops, you'll master for, while, do-while, and foreach — the tools that let PHP do the heavy lifting.