Question

I have a file which stores some value. Users can add stuff to that file and the counter in that file is updated. But if two users open the file, they’ll get the same counter ($arr['counter']). What should I do? Maybe can I lock the file for one user and release the lock after he updates the counter and add some stuff back to the file? Or PHP already locks the file once is opened and I don’t need to worry? Here’s my current code:

    $handle = fopen($file, 'r');
    $contents = fread($handle, filesize($file));
    fclose($handle);       

    $arr = json_decode($contents);

    //Add stuff here to $arr and update counter $arr['counter']++

    $handle = fopen($file, 'w');
    fwrite($handle, json_encode($arr));   
    fclose($handle);      

Answer 1

PHP has the flock function which will lock the file before writing to it, example,

$handle = fopen($file, 'r');
$contents = fread($handle, filesize($file));
fclose($handle);       

$arr = json_decode($contents);

//Add stuff here to $arr and update counter $arr['counter']++

$handle = fopen($file, 'w');
if(flock($handle, LOCK_EX))
{
    fwrite($handle, json_encode($arr));
    flock($handle, LOCK_UN);        
}
else
{
    // couldn't lock the file
}
fclose($handle);