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How do you read the contents of a file in PHP using functions like file_get_contents and fopen? How do you check whether a file or directory exists, and how do you retrieve useful metadata like a file's size or last modified time? How do you work with directories in PHP, including listing their contents, creating new directories, and recursively deleting them? How do file locking mechanisms in PHP, using the flock function, help prevent data corruption when multiple processes might write to the same file concurrently? How do stream wrappers and stream contexts in PHP allow file functions to work seamlessly with non local resources like remote URLs or cloud storage? What security risks are associated with file handling operations that involve user supplied file paths, and how does path traversal attack prevention work?
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How do you write data to a file in PHP, and what is the difference between overwriting a file and appending to it?

Beginner

Answer

file_put_contents writes data to a file, completely overwriting any existing content by default, while passing the FILE_APPEND flag instead adds the new content to the end of the existing file without removing what was already there, and the fopen function similarly supports different modes, such as w for overwriting and a for appending, giving you explicit control over exactly how your write operation should behave.
file_put_contents('log.txt', 'New entry' . PHP_EOL, FILE_APPEND);
Real-world example A logging system appends each new log entry to the end of an existing log file using the FILE_APPEND flag, ensuring previous log history is always preserved rather than being overwritten every time a new entry is recorded.

Common follow-ups: What happens if you try to write to a file that does not yet exist?;How do file permissions affect whether PHP can successfully write to a specific file?

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All 7 File Handling & File System Functions questions