PHP Arrays & File Handling: Syntax, Operations & Security

TAKEAWAYS:

• PHP arrays store multiple values in a single variable using indexed or associative keys, and support nested structures for complex data.
• File handling in PHP involves opening, reading, writing, and closing files using functions like fopen(), fread(), fwrite(), and fclose().
• Error handling with @ or try-catch prevents crashes when files are missing or permissions fail.
• Security risks like directory traversal attacks can be mitigated by validating file paths and using basename().
• Real-world uses include processing user uploads (e.g., Daraz product images), logging system activities (e.g., Ncell transaction records), and generating dynamic reports (e.g., NEPSE stock data).
• Multidimensional arrays and file operations enable efficient data storage and retrieval in web applications.

PHP Arrays: Types, Syntax, and Operations

1. Types of Arrays

PHP supports three main array types, each with unique use cases:

classDiagram
    class IndexedArray {
        +keys: 0, 1, 2, ...
        +values: Any data type
        +Example: $fruits = ["apple", "banana"]
    }
    class AssociativeArray {
        +keys: Named strings
        +values: Any data type
        +Example: $person = ["name" => "Rohan", "age" => 25]
    }
    class MultidimensionalArray {
        +keys: Nested arrays
        +values: Arrays or scalars
        +Example: $matrix = [[1, 2], [3, 4]]
    }
    IndexedArray --> "Uses" Array
    AssociativeArray --> "Uses" Array
    MultidimensionalArray --> "Uses" Array

Indexed Arrays

• Use numeric keys (starting at 0).
• Ideal for lists where order matters (e.g., shopping cart items).
• Example:

  $colors = ["red", "green", "blue"];
  echo $colors[1]; // Output: green
  

Associative Arrays

• Use string keys (e.g., "name", "email").
• Perfect for structured data like user profiles or configuration settings.
• Example:

  $user = ["id" => 101, "name" => "Priya", "role" => "admin"];
  echo $user["name"]; // Output: Priya
  

Multidimensional Arrays

• Arrays inside arrays, forming tables or trees.
• Used for complex data like nested comments or hierarchical menus.
• Example:

  $students = [
      ["name" => "Rohan", "marks" => 85],
      ["name" => "Sita", "marks" => 92]
  ];
  echo $students[1]["name"]; // Output: Sita
  

2. Common Array Functions

PHP provides built-in functions to manipulate arrays efficiently:

Worked Example: Shopping Cart Imagine an e-commerce site like Daraz where users add products to a cart. A multidimensional array tracks items, quantities, and prices:

$cart = [
    ["product" => "Laptop", "quantity" => 1, "price" => 50000],
    ["product" => "Mouse", "quantity" => 2, "price" => 500]
];
$total = 0;
foreach ($cart as $item) {
    $total += $item["quantity"] * $item["price"];
}
echo "Total: ₹" . $total; // Output: ₹51000

3. Looping Through Arrays

Use loops to iterate over arrays and perform operations:

flowchart TD
    A["Start"] --> B["Initialize loop\n(e.g., foreach)"]
    B --> C["Access current element\n(e.g., $value)"]
    C --> D["Process data\n(e.g., echo, calculate)"]
    D --> E["Check loop condition"]
    E -->|"True"| C
    E -->|"False"| F["End"]

foreach Loop

Best for reading array elements:

$fruits = ["apple", "banana", "cherry"];
foreach ($fruits as $fruit) {
    echo $fruit . "<br>";
}
// Output:
// apple
// banana
// cherry

for Loop

Useful when you need the index:

for ($i = 0; $i < count($fruits); $i++) {
    echo "Index " . $i . ": " . $fruits[$i] . "<br>";
}

Worked Example: NEPSE Stock Data A financial dashboard might display stock prices using an associative array:

$stocks = [
    "NTC" => 120.50,
    "Ncell" => 850.75,
    "NMB" => 320.25
];
echo "<h3>Today's Stock Prices</h3>";
foreach ($stocks as $company => $price) {
    echo "$company: ₹$price<br>";
}

File Handling in PHP

File handling allows PHP to interact with files on the server, enabling tasks like logging, configuration, and user uploads.

1. File Operations

PHP uses file handles to read/write files. Key functions:

Modes for fopen():

• "r": Read-only (fails if file doesn’t exist).
• "w": Write-only (creates file if it doesn’t exist; erases content).
• "a": Append-only (creates file if it doesn’t exist; adds to end).
• "r+": Read and write.
• "x": Write-only (fails if file exists).

2. Reading a File

Example: Log File Analysis A bank like Nabil Bank might log transactions in transactions.log. To read it:

$logFile = fopen("transactions.log", "r");
if ($logFile) {
    while (!feof($logFile)) { // Loop until end of file
        $line = fgets($logFile);
        echo htmlspecialchars($line) . "<br>";
    }
    fclose($logFile);
} else {
    echo "Error: Could not open log file.";
}

3. Writing to a File

Example: User Uploads (e.g., Daraz Product Images) When a user uploads an image, PHP saves it to a directory:

$uploadDir = "uploads/";
$fileName = basename($_FILES["image"]["name"]);
$targetPath = $uploadDir . $fileName;

if (move_uploaded_file($_FILES["image"]["tmp_name"], $targetPath)) {
    file_put_contents("upload_log.txt", date("Y-m-d H:i:s") . " - " . $fileName . " uploaded\n", FILE_APPEND);
    echo "File uploaded successfully!";
} else {
    echo "Error uploading file.";
}

4. Error Handling

Always validate file operations to avoid crashes:

Method 1: @ Operator (Suppress Errors)

$file = @fopen("nonexistent.txt", "r");
if (!$file) {
    echo "File not found!";
}

Method 2: try-catch (Exception Handling)

try {
    if (!file_exists("config.ini")) {
        throw new Exception("Config file missing!");
    }
    $config = parse_ini_file("config.ini");
} catch (Exception $e) {
    echo "Error: " . $e->getMessage();
}

Security Risks and Best Practices

1. Directory Traversal Attacks

Attackers exploit poorly validated file paths to access sensitive files (e.g., /etc/passwd). Mitigation:

• Use basename() to strip directory paths:

  $safeFile = basename($_FILES["upload"]["name"]);
  

• Restrict uploads to specific directories:

  $allowedTypes = ["jpg", "png", "gif"];
  $fileExt = strtolower(pathinfo($safeFile, PATHINFO_EXTENSION));
  if (!in_array($fileExt, $allowedTypes)) {
      die("Invalid file type!");
  }
  

2. File Permissions

Ensure PHP has write permissions for directories:

chmod 755 uploads/  # Owner: read/write/execute; Others: read/execute

3. Sanitizing File Content

Use htmlspecialchars() when displaying file content to prevent XSS:

echo htmlspecialchars(file_get_contents("data.txt"));

In the Real World

1. eSewa and Khalti (Payment Processing)

   • Idea Used: Associative Arrays
   • How: When a user pays for a bill (e.g., NTC or NEPSE), the system stores transaction details in an associative array:

     $transaction = [
         "user_id" => 12345,
         "amount" => 500,
         "service" => "NTC Bill",
         "status" => "completed",
         "timestamp" => "2024-05-20 14:30:00"
     ];
     

   • The array is later logged to a file (transactions_2024-05.log) for record-keeping.

2. Pathao (Ride Queue Management)

   • Idea Used: Multidimensional Arrays + File Handling
   • How: Pathao’s backend maintains a queue of ride requests as a multidimensional array:

     $rideQueue = [
         ["user_id" => 101, "pickup" => "Kathmandu", "destination" => "Lalitpur", "time" => "15:45"],
         ["user_id" => 102, "pickup" => "Bhaktapur", "destination" => "Nepalgunj", "time" => "16:00"]
     ];
     

   • When a driver accepts a ride, the entry is removed from the array and logged to ride_logs.txt for analytics.

3. Daraz (Product Inventory)

   • Idea Used: Indexed Arrays + File Handling
   • How: Daraz stores product inventory in a large indexed array:

     $inventory = [
         0 => ["id" => "P1001", "name" => "Smartphone", "stock" => 50, "price" => 25000],
         1 => ["id" => "P1002", "name" => "Laptop", "stock" => 30, "price" => 80000]
     ];
     

   • When a user places an order, the stock is updated in the array, and the change is written to inventory_backup.csv for recovery.

Exam Tip

1. Arrays:

   • Memorize the syntax for indexed, associative, and multidimensional arrays.
   • Practice looping (foreach, for) and common functions (count(), array_push(), in_array()).
   • Exam Question Type: Given an array, write code to:
     • Add/remove elements.
     • Search for a value.
     • Calculate totals (e.g., cart subtotal).

2. File Handling:

   • Know the modes of fopen() ("r", "w", "a") and their effects.
   • Understand error handling (@, try-catch, file_exists()).
   • Exam Question Type: Write code to:
     • Read a file line by line.
     • Append data to a log file.
     • Validate and sanitize user uploads.

3. Security:

   • Always validate file paths (basename()).
   • Restrict file permissions (e.g., chmod 755).
   • Exam Question Type: Identify vulnerabilities in given code (e.g., directory traversal) and suggest fixes.

4. Real-World Scenarios:

   • Expect questions linking arrays/files to e-commerce (Daraz), payments (eSewa), or logging (Ncell).
   • Example: "How would you store and retrieve user orders in a PHP array? Write code to log orders to a file."

Visual Summary:

mindmap
  root((PHP Arrays & File Handling))
    Arrays
      Indexed
      Associative
      Multidimensional
    File Operations
      fopen()
      fread()/fwrite()
      file_get_contents()
    Security
      Directory Traversal
      Permission Checks
    Real-World
      eSewa Transactions
      Daraz Inventory
      Pathao Ride Queue