Example 5: Traffic Light System

Conditional structures elseif in PHP

Download file semáforo

🎯 Live Demo

Traffic Light Simulator

Color: Verde

You can go!

Change the traffic light color!

Quick change:

Automatic mode:

The traffic light will change automatically every 2 seconds

📖 Code Explanation

Complete PHP Code

<?php
$color = "verde";

if ($color == "verde") {
    echo "You can go!";
} elseif ($color == "amarillo") {
    echo "Caution!";
} elseif ($color == "rojo") {
    echo "Stop! Wait...";
} else {
    echo "Invalid color";
}
?>

📝 Line 1: Variable declaration

$color = "verde";

What does it do? Creates a string variable that stores the current traffic light color.

Why is it done this way? Colors are text, so they are assigned in quotes. We use lowercase for consistency.

💡 Concept: String comparison

In PHP, strings are compared with == or ===. It's case-sensitive, that's why we use strtolower() to normalize.

📝 Lines 3-11: if-elseif-else structure

if ($color == "verde") {
echo "You can go!";
} elseif ($color == "amarillo") {
echo "Caution!";
} elseif ($color == "rojo") {
echo "Stop! Wait...";
} else {
echo "Invalid color";
}

What does it do? Evaluates multiple conditions in sequence until it finds a true one.

Why is it done this way? elseif allows handling multiple specific cases before the default case.

💡 Concept: Flow of elseif

PHP evaluates conditions in order:
1. If the first is TRUE → executes its block
2. If it's FALSE → moves to the next condition
3. If none is TRUE → executes the block else

📝 Differences with switch

With elseif

if ($color == "verde") {
    // código verde
} elseif ($color == "amarillo") {
    // código amarillo
} else {
    // código por defecto
}

With switch

switch ($color) {
    case "verde":
        // código verde
        break;
    case "amarillo":
        // código amarillo
        break;
    default:
        // código por defecto
}

elseif is better when you need complex conditions. switch is better for simple equality comparisons.

🔄 Multiple Control Structures

if-elseif-else

if ($cond1) {
    // código 1
} elseif ($cond2) {
    // código 2
} else {
    // código 3
}

✅ Advantages:

Complex conditions
Custom evaluation
Total flexibility

switch-case

switch ($variable) {
    case "valor1":
        // código 1
        break;
    case "valor2":
        // código 2
        break;
    default:
        // código default
}

✅ Advantages:

More readable
Better for simple equality checks
Multiple equal cases

🎯 Common Use Cases

✅ State Systems

Managing states such as: active/inactive/pending

if ($estado == "activo") {
    // usuario activo
} elseif ($estado == "inactivo") {
    // usuario inactivo
} elseif ($estado == "pendiente") {
    // usuario pendiente
}

✅ Access Levels

Permission control: admin/editor/user

if ($nivel == "admin") {
    // acceso total
} elseif ($nivel == "editor") {
    // acceso edición
} else {
    // acceso básico
}

✅ Scoring Systems

Classification by range: A/B/C/D

if ($puntos >= 90) {
    $grado = "A";
} elseif ($puntos >= 80) {
    $grado = "B";
} elseif ($puntos >= 70) {
    $grado = "C";
} else {
    $grado = "D";
}

🎯 Key Concepts Learned

Multiple Elseif

Handles more than two conditions
Evaluated in sequential order
Only one block runs
Useful for categories

String Comparison

Case-sensitive by default
Use strtolower()
== vs === for comparison
Important for validation

Default Case

Handles unexpected cases
Provides robustness
Improves user experience
Prevents errors

💪 Practical Exercise

Extend the traffic light system

Practice these changes to improve your skills:

Add a color "blue" for "proceed with extreme caution"
Implement a traffic light system for pedestrians (green/red)
Create a smart traffic light that suggests an action based on the time
Convert the code to a switch structure and compare
Add validation for uppercase and lowercase colors

Solution 1: Color blue added

<?php
if ($color == "verde") {
    echo "You can go!";
} elseif ($color == "amarillo") {
    echo "Caution!";
} elseif ($color == "rojo") {
    echo "Stop! Wait...";
} elseif ($color == "azul") {
    echo "Proceed with extreme caution";
} else {
    echo "Invalid color";
}
?>

Solution 2: Traffic Light pedestrian

<?php
if ($color == "verde") {
    echo "🟢 Pedestrian: Cross now";
} elseif ($color == "rojo") {
    echo "🔴 Pedestrian: Wait to cross";
} else {
    echo "Invalid color for pedestrians";
}
?>

Solution 3: Traffic Light smart

<?php
$hora = date('H');
if ($hora >= 6 && $hora <= 22) {
    // Horario normal
    if ($color == "verde") {
        echo "You can go!";
    } elseif ($color == "rojo") {
        echo "Stop! Wait...";
    }
} else {
    // Horario nocturno
    if ($color == "verde") {
        echo "You can go with caution!";
    } elseif ($color == "rojo") {
        echo "Stop! Traffic light in night mode";
    }
}
?>

Solution 4: With switch

<?php
switch ($color) {
    case "verde":
        echo "You can go!";
        break;
    case "amarillo":
        echo "Caution!";
        break;
    case "rojo":
        echo "Stop! Wait...";
        break;
    default:
        echo "Invalid color";
}
?>

Solution 5: Validation case-insensitive

<?php
$color = strtolower($color); // Normalize to lowercase
if ($color == "verde") {
    echo "You can go!";
} elseif ($color == "amarillo") {
    echo "Caution!";
} elseif ($color == "rojo") {
    echo "Stop! Wait...";
} else {
    echo "Invalid color";
}
?>