PHP Simultaneous file access / flock() issue

Question

I'm having trouble implementing a script which should parse a json string from a file, either overwrite the object with the same id or add it to the json array and write it back to the file. The script is called in a for loop so that I tried to use flock to prevent overwriting the file before the json string could be parsed, but the results look really strange and I don't know whats going wrong. Here is the script:

$method = $_SERVER['REQUEST_METHOD'];

$file = fopen($userid . '.json', 'c+');
flock($file, LOCK_EX);

$jsonStr = (filesize($userid . '.json') == 0 ) ? '{"success": true}' : fread($file, filesize($userid . '.json'));
$jsonObj = json_decode($jsonStr);

if($method === 'POST') {
$dataStr = file_get_contents('php://input');
$dataObj = json_decode($dataStr);

$replacements = array();
if(isset($jsonObj->images)) {
    foreach($jsonObj->images as $idx=>$image) {
            if($image->id == $dataObj->id) {
                $replacements += array($idx => $dataObj);
            }
    }
    if(count($replacements)==0) {
        array_push($replacements, $dataObj);
    }
    $jsonObj->images = array_replace($jsonObj->images, $replacements);
} else {
    //$jsonObj = new stdClass();
    $jsonObj->images = new stdClass(); 
    $jsonObj->images = array($dataObj);
}


fwrite($file, json_encode($jsonObj));
file_put_contents('log.json', json_encode($dataObj), FILE_APPEND);

echo '{"success":true, "images":' . $dataStr . '}';
} else if($method === 'GET') {
echo $jsonStr;
}

fclose($file);

As result I get sometime three files one just called ".json" and the other one correctly userid.json and the log.json. The log looks best, since all 26 objects are shown correctly, but without checking for exiting objects previously. The ".json" is just blank and the looks really odd:

{"success":true,"images":[{"id":"f9f4b6ee-8b7b-414e-98f4-47ed5ee3c594","top":1200,"left":4050,"clicks":0,"affinity":[]}]}
{"success":true,"images":[{"id":"034c7661-9466-4651-b860-7e049e1543ac","top":900,"left":600,"clicks":0,"affinity":[]}]}
{"images":[{"id":"eebc35ec-6c0e-416a-9516-061df821083d","top":1800,"left":3300,"clicks":0,"affinity":[]}]}
{"images":[{"id":"e09da5b9-2e65-4a12-94cd-4745b354a256","top":1200,"left":1200,"clicks":0,"affinity":[]}]}
{"images":[{"id":"b023a259-4554-48a5-acd8-cb4215e6a391","top":1350,"left":1500,"clicks":0,"affinity":[]}]}
{"images":[{"id":"6521d8c3-461c-4fcd-b69d-69f14ae37418","top":600,"left":3750,"clicks":0,"affinity":[]}]}
{"images":[{"id":"4c082c1a-d4ae-4c1c-bfc4-ed36980f5db2","top":3150,"left":600,"clicks":0,"affinity":[]}]}
{"images":[{"id":"22dbdea2-4936-4353-825e-9af6e6f2ca93","top":3000,"left":2100,"clicks":0,"affinity":[]}]}
{"images":[{"id":"a7ba32d4-912e-489d-8f9b-e412bc1629c1","top":2850,"left":3750,"clicks":0,"affinity":[]}]}
{"images":[{"id":"b258148d-1779-4f17-ae5e-1160aa0a34b9","top":750,"left":4050,"clicks":0,"affinity":[]}]}
{"images":[{"id":"21c1c9d1-a7a6-48d2-a4ba-174c0fe248a3","top":1650,"left":750,"clicks":0,"affinity":[]}]}
{"images":[{"id":"bfed2f56-7876-4eb2-a217-e0f71f4455ee","top":3300,"left":1500,"clicks":0,"affinity":[]}]}
{"images":[{"id":"7f13b674-5d07-444f-bf6d-463758c96788","top":1650,"left":1950,"clicks":0,"affinity":[]}]}
{"images":[{"id":"fb8711f2-ba10-44d7-9038-02cae6438506","top":1800,"left":450,"clicks":0,"affinity":[]}]}
{"images":[{"id":"ac83e869-e4fb-4eee-8d2e-4d26762101d9","top":750,"left":1800,"clicks":0,"affinity":[]}]}
{"images":[{"id":"069a7595-2271-4430-b324-974115df7b80","top":2550,"left":1800,"clicks":0,"affinity":[]}]}
{"images":[{"id":"ea9ab2fc-e179-4f56-9ce7-8af456911b33","top":1950,"left":750,"clicks":0,"affinity":[]}]}
{"images":[{"id":"9b997ae3-0c66-4f78-937a-2fde2470d7d7","top":300,"left":300,"clicks":0,"affinity":[]}]}
{"images":[{"id":"f860dea4-3b27-4401-999b-72ce45022110","top":1200,"left":1950,"clicks":0,"affinity":[]}]}
{"images":[{"id":"2633c107-8a51-4e9c-bc2c-f87c30f7359d","top":750,"left":2100,"clicks":0,"affinity":[]}]}
{"images":[{"id":"bebcb2bb-c2f0-4e4d-b202-b05090eacf49","top":3600,"left":3600,"clicks":0,"affinity":[]}]}
{"images":[{"id":"460f961e-f1dd-4329-95be-d30770a37b83","top":2400,"left":1200,"clicks":0,"affinity":[]}]}
{"images":[{"id":"ee9b254f-fe6a-4ece-a662-e934625e992a","top":3600,"left":600,"clicks":0,"affinity":[]}]}
{"images":[{"id":"4e047ea9-345e-4cf9-a066-d0b431820c61","top":2100,"left":1350,"clicks":0,"affinity":[]}]}
{"images":[{"id":"8bbb5327-6d39-40e1-919e-d7b70e13fc2b","top":2700,"left":1200,"clicks":0,"affinity":[]}]}
{"images":[{"id":"0c50c0ec-e18b-4917-9787-bad245eed798","top":1200,"left":2550,"clicks":0,"affinity":[]}]}

I also get an exception:

<b>Strict Standards</b>:  Creating default object from empty value in <b>C:\xampp\htdocs\gallery\store.php</b> on line <b>38</b><br />

I guess I didn't understand the procedure correctly and I'm happy for any help.

EDIT: The result I want, looks like this:

{"success":true,"images":[{"id":"f9f4b6ee-8b7b-414e-98f4-47ed5ee3c594","top":1200,"left":4050,"clicks":0,"affinity":[]},
{"id":"034c7661-9466-4651-b860-7e049e1543ac","top":900,"left":600,"clicks":0,"affinity":[]},{"id":"eebc35ec-6c0e-416a-9516-061df821083d","top":1800,"left":3300,"clicks":0,"affinity":[]},
{"id":"e09da5b9-2e65-4a12-94cd-4745b354a256","top":1200,"left":1200,"clicks":0,"affinity":[]},
{"id":"b023a259-4554-48a5-acd8-cb4215e6a391","top":1350,"left":1500,"clicks":0,"affinity":[]},
{"id":"6521d8c3-461c-4fcd-b69d-69f14ae37418","top":600,"left":3750,"clicks":0,"affinity":[]},
{"id":"4c082c1a-d4ae-4c1c-bfc4-ed36980f5db2","top":3150,"left":600,"clicks":0,"affinity":[]},
{"id":"22dbdea2-4936-4353-825e-9af6e6f2ca93","top":3000,"left":2100,"clicks":0,"affinity":[]},
{"id":"a7ba32d4-912e-489d-8f9b-e412bc1629c1","top":2850,"left":3750,"clicks":0,"affinity":[]},
{"id":"b258148d-1779-4f17-ae5e-1160aa0a34b9","top":750,"left":4050,"clicks":0,"affinity":[]},
{"id":"21c1c9d1-a7a6-48d2-a4ba-174c0fe248a3","top":1650,"left":750,"clicks":0,"affinity":[]},
{"id":"bfed2f56-7876-4eb2-a217-e0f71f4455ee","top":3300,"left":1500,"clicks":0,"affinity":[]},
{"id":"7f13b674-5d07-444f-bf6d-463758c96788","top":1650,"left":1950,"clicks":0,"affinity":[]},
{"id":"fb8711f2-ba10-44d7-9038-02cae6438506","top":1800,"left":450,"clicks":0,"affinity":[]},
{"id":"ac83e869-e4fb-4eee-8d2e-4d26762101d9","top":750,"left":1800,"clicks":0,"affinity":[]},
{"id":"069a7595-2271-4430-b324-974115df7b80","top":2550,"left":1800,"clicks":0,"affinity":[]},
{"id":"ea9ab2fc-e179-4f56-9ce7-8af456911b33","top":1950,"left":750,"clicks":0,"affinity":[]},
{"id":"9b997ae3-0c66-4f78-937a-2fde2470d7d7","top":300,"left":300,"clicks":0,"affinity":[]},
{"id":"f860dea4-3b27-4401-999b-72ce45022110","top":1200,"left":1950,"clicks":0,"affinity":[]},
{"id":"2633c107-8a51-4e9c-bc2c-f87c30f7359d","top":750,"left":2100,"clicks":0,"affinity":[]},
{"id":"bebcb2bb-c2f0-4e4d-b202-b05090eacf49","top":3600,"left":3600,"clicks":0,"affinity":[]},
{"id":"460f961e-f1dd-4329-95be-d30770a37b83","top":2400,"left":1200,"clicks":0,"affinity":[]},
{"id":"ee9b254f-fe6a-4ece-a662-e934625e992a","top":3600,"left":600,"clicks":0,"affinity":[]},
{"id":"4e047ea9-345e-4cf9-a066-d0b431820c61","top":2100,"left":1350,"clicks":0,"affinity":[]},
{"id":"8bbb5327-6d39-40e1-919e-d7b70e13fc2b","top":2700,"left":1200,"clicks":0,"affinity":[]},
{"id":"0c50c0ec-e18b-4917-9787-bad245eed798","top":1200,"left":2550,"clicks":0,"affinity":[]}]}

Cheers, Daniel

Answer 1

I finally have it working and for anyone else having issues with simultaneous file access, here's my solution:

1. Check if the file exists, if not, create it and set up the data structure (to prevent the strict standard error)

   $filename = $userid . '.json';
   if(!file_exists($filename)) {
       $file = fopen($filename, 'w');
       if(flock($file, LOCK_EX)) {
           fwrite($file, '{"success": true, "images": []}');
           flock($file, LOCK_UN);
           fclose($file);
       }
   }
   

2. Open the file using c+ as second parameter (w+ would set the filesize to 0). Check if you can get the exclusive lock. Read the file and rewind in order to overwrite it later.

   else {
       $file = fopen($userid . '.json', 'c+');
       if(flock($file, LOCK_EX)) {
           $jsonStr = fread($file, filesize($userid . '.json'));
           $jsonObj = json_decode($jsonStr);
           rewind($file);
   

3. Manipulate the object how you need it and write it back as encoded json string using fwrite

           if($method === 'POST') {
               $dataStr = file_get_contents('php://input');
               if(isset($dataStr) &&$dataStr != '') {
                   $dataObj = json_decode($dataStr);
                   $replace = -1;
                   foreach($jsonObj->images as $idx=>$image) {
                       if($image->id == $dataObj->id) {
                           $replace = $idx;
                       }
                   }
                   if($replace != -1) {
                       $img = $jsonObj->images;
                       $img[$replace] = $dataObj;
                       $jsonObj->images = $img;
                   } else {
                       array_push($jsonObj->images, $dataObj);
                   }
                   fwrite($file, json_encode($jsonObj));
               }
               echo '{"success":true, "images":' . $dataStr . '}';
           } else if($method === 'GET') {
               echo $jsonStr;
           }
   

4. Release the lock and close the file.

           flock($file, LOCK_UN);
           fclose($file);
       }
   }