PHP: Error Handling and Debugging

Notice      ← "You may have overlooked something" (undefined variable, offset)
Warning     ← "The code has a problem but can continue" (include file not found)
Fatal Error ← "The code has a serious error and must stop" (calling a non-existent function)

<?php
// Notice: undefined variable (PHP continues execution)
echo $undefinedVar;  // Notice: Undefined variable $undefinedVar

// Warning: file not found (PHP continues execution)
include 'nonexistent.php';  // Warning: Failed opening...

// Fatal Error: calling a non-existent function (PHP stops)
thisFunctionDoesNotExist();  // Fatal Error: Call to undefined function
?>

<?php
// Development: show all errors
error_reporting(E_ALL);
ini_set('display_errors', '1');

// Production: log but don't display
error_reporting(E_ALL);
ini_set('display_errors', '0');
ini_set('log_errors', '1');
ini_set('error_log', __DIR__ . '/php-errors.log');
?>

<?php
$isDev = ($_SERVER['SERVER_NAME'] ?? '') === 'localhost';

if ($isDev) {
    error_reporting(E_ALL);
    ini_set('display_errors', '1');
} else {
    error_reporting(E_ALL);
    ini_set('display_errors', '0');
    ini_set('log_errors', '1');
    ini_set('error_log', '/var/log/php/errors.log');
}
?>

<?php
function divide(float $a, float $b): float {
    if ($b === 0.0) {
        throw new Exception("Division by zero is not allowed!");
    }
    return $a / $b;
}

try {
    echo divide(10, 2) . "<br>";  // 5
    echo divide(10, 0) . "<br>";  // Throws an exception
} catch (Exception $e) {
    echo "[Caught Exception] {$e->getMessage()}<br>";
    echo "File: {$e->getFile()}<br>";
    echo "Line: {$e->getLine()}<br>";
} finally {
    echo "Whether success or failure, finally always runs (e.g., close files, release resources)<br>";
}
?>

<?php
class ValidationException extends Exception {
    private array $errors;
    
    public function __construct(array $errors, string $message = "Validation failed") {
        parent::__construct($message);
        $this->errors = $errors;
    }
    
    public function getErrors(): array {
        return $this->errors;
    }
}

class UserNotFoundException extends Exception {}

// Using custom exceptions
function findUser(int $id): array {
    $users = [1 => ['name' => 'John'], 2 => ['name' => 'Jane']];
    
    if (!isset($users[$id])) {
        throw new UserNotFoundException("User ID {$id} does not exist");
    }
    return $users[$id];
}

function validateUsername(string $name): void {
    $errors = [];
    if ($name === '') $errors[] = 'Username cannot be empty';
    if (strlen($name) > 50) $errors[] = 'Username is too long';
    if (!empty($errors)) throw new ValidationException($errors);
}

// Unified exception handling
try {
    validateUsername('');
    $user = findUser(99);
} catch (ValidationException $e) {
    echo "[Validation Failed]<br>" . implode("<br>", $e->getErrors());
} catch (UserNotFoundException $e) {
    echo "[User Not Found] {$e->getMessage()}";
} catch (Exception $e) {
    echo "[Unknown Error] {$e->getMessage()}";
}
?>

<?php
class AppLogger {
    public function __construct(
        private string $logDir
    ) {
        if (!is_dir($logDir)) {
            mkdir($logDir, 0755, true);
        }
    }
    
    public function info(string $message, array $context = []): void {
        $this->log('INFO', $message, $context);
    }
    
    public function error(string $message, array $context = []): void {
        $this->log('ERROR', $message, $context);
    }
    
    private function log(string $level, string $message, array $context): void {
        $date = date('Y-m-d');
        $time = date('H:i:s');
        
        $line = "[{$time}] [{$level}] {$message}";
        if (!empty($context)) {
            $line .= ' ' . json_encode($context, JSON_UNESCAPED_UNICODE);
        }
        $line .= "\n";
        
        $file = "{$this->logDir}/app-{$date}.log";
        file_put_contents($file, $line, FILE_APPEND | LOCK_EX);
    }
}

$log = new AppLogger(__DIR__ . '/logs');
$log->info('User logged in', ['username' => 'John', 'ip' => $_SERVER['REMOTE_ADDR'] ?? '']);
$log->error('Payment failed', ['order_id' => 12345, 'reason' => 'Insufficient balance']);
?>

<?php
// Most commonly used debug output
$data = ['name' => 'John', 'skills' => ['PHP', 'MySQL']];

var_dump($data);  // Detailed output (type + value)
// array(2) { ["name"]=> string(4) "John" ["skills"]=> ... }

print_r($data);   // Concise output
// Array ( [name] => John [skills] => Array ( [0] => PHP [1] => MySQL ) )

// Formatted var_dump (recommended)
echo '<pre>'; print_r($data); echo '</pre>';

// Debug and terminate
var_dump($data); exit;  // or die(var_dump($data));
?>

<?php
// Value + type
var_dump($variable);

// Only show in development
if ($isDev) {
    echo '<pre>'; var_dump($data); echo '</pre>';
}

// Log it
error_log("Debug info: " . print_r($data, true));

// Check if execution reaches a certain line
echo "HERE"; exit;  // or error_log("Reached this line");

// View the call stack
debug_print_backtrace();

// Check if a variable is set
var_dump(isset($var), empty($var));
?>

<?php
// Output buffering: capture echo/print output
ob_start();
echo "This text won't be sent directly to the browser.";
$content = ob_get_clean();  // Get and clear the buffer
// $content is now "This text won't be sent directly to the browser."

// Custom error handler
function myErrorHandler(int $errno, string $errstr, string $errfile, int $errline): bool {
    $message = "[{$errno}] {$errstr} in {$errfile}:{$errline}";
    error_log($message);
    
    if (ini_get('display_errors')) {
        echo "<div style='background:#ffebee;padding:10px;margin:10px'>
            <strong>PHP Error:</strong> {$message}
        </div>";
    }
    return true;  // Prevent PHP's default handling
}
set_error_handler('myErrorHandler');

// Test the custom error handler
echo $undefined;  // Now displays in a nice HTML box
?>