The complete one!

LockHandlerUtil.php

/**
 * Class LockHandlerUtil
 */
class LockHandlerUtil
{
    /**
     * @param String $fileName
     * @param String $rootDir
     */
    public function __construct($fileName, $rootDir)
    {
        $this->fileName = $fileName;
        $this->pidFolderPath = $rootDir.'/../pid_files/';
    }
    
    /**
     * @return boolean
     */
    public function isDuplicateInstanceRunning()
    {
        $filePath = $this->getFilePath();
        $fileExists = file_exists($filePath);
        if ($fileExists) {
            $pid = file_get_contents($filePath);
            if (file_exists("/proc/".$pid)) {
                return true;
            }
        }
        $this->createPIDFile();
        
        return false;
    }
    
    /**
     * @return null
     */
    public function releaseInstance()
    {
        $filePath = $this->getFilePath();
        unlink($filePath);
    }
    
    /**
     * @return null
     */
    private function createPIDFile()
    {
        file_put_contents($this->getFilePath(), (string) $this->getProcessId(), LOCK_EX);
    }
    
    /**
     * @return Integer
     */
    private function getProcessId()
    {
        return getmypid();
    }
    
    /**
     * @return String
     */
    private function getFilePath()
    {
        return $this->pidFolderPath.$this->fileName.".pid";
    }
}