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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 write data to a file in PHP, and what is the difference between overwriting a file and appending to it? 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 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 file locking mechanisms in PHP, using the flock function, help prevent data corruption when multiple processes might write to the same file concurrently?

Intermediate

Answer

The flock function lets you acquire an exclusive or shared lock on a file before reading from or writing to it, preventing race conditions where multiple processes or requests attempting to write to the same file simultaneously could corrupt its contents or produce inconsistent results, which is especially important for applications that use file based storage for things like counters or logs that might be accessed concurrently by multiple simultaneous requests.
$handle = fopen('counter.txt', 'c+');
if (flock($handle, LOCK_EX)) {
    $count = (int) fread($handle, 1024);
    ftruncate($handle, 0);
    rewind($handle);
    fwrite($handle, $count + 1);
    flock($handle, LOCK_UN);
}
fclose($handle);
Real-world example A simple visit counter stored as a text file uses flock to acquire an exclusive lock before reading and updating the count, preventing two simultaneous visitors from causing the counter to lose an increment due to a race condition.

Common follow-ups: What is the difference between an exclusive lock and a shared lock?;Why might a database or dedicated cache be a better solution than file based locking for high traffic scenarios?

PDO & Databases;Caching Strategies in PHP
All 7 File Handling & File System Functions questions