为什么php文件不能独占锁定工作？

I have some code with process forks (pcntl_fork()) and lockfile to distribute tasks between the forks.

I'm checking if a task available for current fork with lockfile using flock():

$pid = getmypid();
$result = false;

$file = fopen($this->lockFile, 'a+b');
do {
    $lock = flock($file, LOCK_EX);
} while (!$lock);

echo "$pid Locked";
fseek($file, 0);
$content = @fread($file, filesize($this->lockFile));
$locked = explode(PHP_EOL, $content);

if (!in_array($package, $locked)) {
    fwrite($file, PHP_EOL . $package);
    fflush($file);
    $result = true;
}

flock($file, LOCK_UN);
fclose($file);
echo "$pid Released";

return $result;

Sometimes I'm getting two output lines with text "Locked" without "Released" line between them. In this case two different forks doing the same task when I want them to do smthng different.