PHP Arrays & File Handling: Structures, Operations & Security

TAKEAWAYS:

• PHP arrays store data in indexed (numeric keys) or associative (string keys) formats, with multidimensional arrays for nested structures.
• File handling in PHP requires proper modes (r, w, a, x) and permissions (chmod) to read/write safely.
• Common array operations include sorting (sort(), asort()), searching (in_array(), array_search()), and merging (array_merge()).
• Security risks like file inclusion vulnerabilities must be mitigated using basename() and realpath().
• Real-world uses include user data storage (e.g., saving form submissions to files) and dynamic content generation (e.g., displaying sorted product lists).

1. PHP Arrays: Types, Declaration, and Operations

Arrays are ordered maps (key-value pairs) in PHP, used to store multiple values in a single variable. They come in three types:

1.1 Types of Arrays

classDiagram
    class IndexedArray {
        +Keys: Numeric (0, 1, 2...)
        +Example: $fruits = ["apple", "banana"]
    }
    class AssociativeArray {
        +Keys: String (e.g., "name", "age")
        +Example: $user = ["name" => "Rohan", "age" => 20]
    }
    class MultidimensionalArray {
        +Nested arrays (arrays within arrays)
        +Example: $students = [ ["Rohan", 20], ["Sita", 21] ]
    }
    IndexedArray <|-- MultidimensionalArray
    AssociativeArray <|-- MultidimensionalArray

1.2 Declaring Arrays

• Indexed Array (numeric keys):

  $fruits = array("Apple", "Banana", "Orange"); // Old syntax
  $fruits = ["Apple", "Banana", "Orange"];     // New syntax (PHP 5.4+)
  

• Associative Array (string keys):

  $user = [
      "name" => "Rohan",
      "age"  => 20,
      "city" => "Kathmandu"
  ];
  

• Multidimensional Array (arrays within arrays):

  $students = [
      ["name" => "Rohan", "age" => 20],
      ["name" => "Sita", "age" => 21]
  ];
  

1.3 Key Array Functions

1.4 Worked Example: Sorting and Displaying Student Data

Task: Create a 2D array of 5 students and display it in an HTML table.

$students = [
    ["name" => "Rohan", "age" => 20, "city" => "Kathmandu"],
    ["name" => "Sita", "age" => 21, "city" => "Pokhara"],
    ["name" => "Hari", "age" => 19, "city" => "Bhaktapur"],
    ["name" => "Gita", "age" => 22, "city" => "Lalitpur"],
    ["name" => "Ramesh", "age" => 20, "city" => "Dharan"]
];

// Sort by age (ascending)
usort($students, function($a, $b) {
    return $a["age"] <=> $b["age"];
});

Output (HTML Table):

<table border="1">
    <tr><th>Name</th><th>Age</th><th>City</th></tr>
    <?php foreach ($students as $student): ?>
        <tr>
            <td><?php echo $student["name"]; ?></td>
            <td><?php echo $student["age"]; ?></td>
            <td><?php echo $student["city"]; ?></td>
        </tr>
    <?php endforeach; ?>
</table>

Trace (After Sorting):

2. File Handling in PHP

Files allow persistent data storage. PHP provides functions to open, read, write, and close files.

2.1 File Opening Modes

Example: Writing to a File (w+ mode)

$file = fopen("sport.txt", "w+");
fwrite($file, "Cricket match today at 3 PM.\nCricket is fun!\n");
fclose($file);

State After Writing:

sport.txt:
Cricket match today at 3 PM.
Cricket is fun!

2.2 Reading a File

Task: Print the portion of sport.txt between the first two occurrences of "cricket".

$file = fopen("sport.txt", "r");
$content = fread($file, filesize("sport.txt"));
fclose($file);

$lines = explode("\n", $content);
$firstCricket = strpos(strtolower($lines[0]), "cricket");
$secondCricket = strpos(strtolower($lines[1]), "cricket");

if ($firstCricket !== false && $secondCricket !== false) {
    echo "Portion between 'cricket':\n";
    echo substr($lines[0], $firstCricket + 8); // +8 to skip "cricket "
    echo "\n" . $lines[1];
}

Output:

Portion between 'cricket':
 match today at 3 PM.
Cricket is fun!

2.3 File Permissions

Check and modify file permissions using:

// Check permissions (returns octal, e.g., 0644)
$permissions = fileperms("sport.txt");
echo "Permissions: " . decoct($permissions); // Output: 0644

// Change permissions (e.g., to 0755)
chmod("sport.txt", 0755);

3. Real-World Applications

In the Real World

1. eSewa (Nepal):

   • Uses associative arrays to store user transaction data (e.g., ["user_id" => 123, "amount" => 500]).
   • File handling logs transactions in transactions.log for auditing.

2. Khalti (Nepal):

   • Multidimensional arrays manage merchant data (e.g., [["merchant_id" => 1, "name" => "Daraz"], ["merchant_id" => 2, "name" => "Pathao"]]).
   • File operations write payment receipts to receipts/{user_id}.txt.

3. NTC Website (Nepal):

   • Array sorting displays train schedules in ascending order (e.g., sort($trains)).
   • File reading loads routes.txt to populate dropdown menus for station selection.

Worked Example: Saving Form Data to a File (Like eSewa/Khalti)

Task: Create a web form to save user data to Person.txt.

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $name = $_POST["name"];
    $age = $_POST["age"];
    $gender = $_POST["gender"];

    $file = fopen("Person.txt", "a");
    fwrite($file, "$name,$age,$gender\n");
    fclose($file);
    echo "Data saved successfully!";
}
?>

HTML Form:

<form method="post">
    Name: <input type="text" name="name"><br>
    Age: <input type="number" name="age"><br>
    Gender:
    <input type="radio" name="gender" value="Male"> Male
    <input type="radio" name="gender" value="Female"> Female<br>
    <input type="submit" value="Submit">
</form>

State After Submission (File Person.txt):

Rohan,20,Male
Sita,21,Female

4. Security Considerations

4.1 File Inclusion Vulnerabilities

Avoid unsafe file operations like:

// UNSAFE: Allows directory traversal attacks
include($_GET["page"] . ".php");

// SAFE: Uses basename() to prevent traversal
include(basename($_GET["page"]) . ".php");

4.2 Generating Unique IDs

Use uniqid() or random_int():

$uniqueId = uniqid("user_", true); // e.g., "user_5f8d3a1b"
$randomId = random_int(1000, 9999); // e.g., 4273

Exam Tip

1. Array Questions:

   • Memorize count(), sort(), asort(), array_search() for sorting/searching.
   • For 2D arrays, use foreach loops to iterate and usort() for custom sorting.

2. File Handling:

   • Modes matter! w erases files; a appends. Always close files with fclose().
   • For file content extraction, use explode() (split lines) and strpos() (find substrings).

3. Common Pitfalls:

   • Off-by-one errors in loops (e.g., for ($i=0; $i<=count($array); $i++) → wrong!).
   • Forgetting fclose() can lead to memory leaks.
   • Unsafe file operations (e.g., include() without basename()) are easy marks for security questions.

Practice Questions:

1. Write a PHP program to merge two associative arrays and display the result.
2. How would you find the length of a multidimensional array?
3. Explain the difference between file_get_contents() and fopen() + fread().
4. Write a function to generate a unique filename (e.g., user_123_20230515.txt).