🎯 PHP File Handling – Interview Questions

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🟢 Basic Level

1. What is file handling in PHP?

👉 Managing files (create, read, write, delete) on the server.

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2. Difference between fread() and fgets()?

👉 fread() → reads full/partial file
👉 fgets() → reads one line at a time

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3. What is the difference between "w" and "a" mode?

👉 "w" → overwrite file
👉 "a" → append data

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4. What does feof() do?

👉 Checks if end of file is reached

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5. What is $_FILES in PHP?

👉 Superglobal used for file uploads

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🟡 Intermediate Level

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6. Explain file upload process in PHP.

👉 Steps:

1. HTML form (enctype="multipart/form-data")
2. Access via $_FILES
3. Move file using move_uploaded_file()

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7. How do you validate file uploads?

👉 Check:

• File type (MIME/extension)
• File size
• Rename file
• Restrict executable files

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8. What is file_get_contents() vs readfile()?

👉 file_get_contents() → returns content
👉 readfile() → outputs directly

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9. What are file permissions in PHP?

👉 Control read/write/execute (e.g., 644, 755)

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10. How do you check if a file exists?

file_exists("file.txt");

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🔴 Advanced Level (Important for Interviews)

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11. What is file locking? Why is it important?

👉 Prevents multiple scripts from writing at same time
👉 Avoids data corruption

flock($file, LOCK_EX);

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12. How do you handle large files efficiently?

👉 Use chunk reading:

while(!feof($file)) {
    echo fread($file, 1024);
}

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13. What are risks in file upload systems?

👉

• Malware upload
• Remote code execution
• File overwrite
• Directory traversal

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14. Difference between unlink() and rmdir()?

👉 unlink() → delete file
👉 rmdir() → delete empty directory

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15. What is directory traversal attack?

👉 Accessing restricted files using paths like:

../../etc/passwd

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💻 Practical Coding Questions

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16. Write code to read a file line by line.

$file = fopen("test.txt", "r");while(!feof($file)) {
    echo fgets($file);
}fclose($file);

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17. Write code to append data into a file.

$file = fopen("test.txt", "a");
fwrite($file, "New Data\n");
fclose($file);

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18. Create a file upload system with validation.

if(isset($_POST['upload'])) {    $fileName = $_FILES['file']['name'];
    $tmp = $_FILES['file']['tmp_name'];
    $ext = pathinfo($fileName, PATHINFO_EXTENSION);    $allowed = ['jpg','png','pdf'];    if(in_array($ext, $allowed)) {
        move_uploaded_file($tmp, "uploads/" . time() . "." . $ext);
        echo "Uploaded";
    } else {
        echo "Invalid file";
    }
}

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19. Write code to list all files in a directory.

$files = scandir("uploads");foreach($files as $file) {
    if($file != "." && $file != "..") {
        echo $file . "<br>";
    }
}

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20. Write code to delete a file safely.

$file = "test.txt";if(file_exists($file)) {
    unlink($file);
    echo "Deleted";
} else {
    echo "File not found";
}

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🧠 Scenario-Based Questions (Very Important)

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21. Your file upload system is getting hacked. What will you do?

👉

• Restrict file types
• Rename files
• Store outside public folder
• Use MIME validation
• Disable script execution

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22. Two users write to same file simultaneously. Problem?

👉 Data corruption
👉 Solution: file locking (flock)

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23. Website becomes slow when reading large files. Fix?

👉 Use chunk reading instead of file_get_contents()

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24. User uploads .php file and executes it. How to prevent?

👉

• Block .php extensions
• Store outside public folder
• Use .htaccess restrictions

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25. How to securely store user-uploaded files?

👉

• Rename file
• Validate MIME type
• Store outside root
• Limit size
• Scan content