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I have couple of php scripts which are called by cron. The cron job runs every 20 mins. Sometimes script finishes the job in few mins and sometimes it takes more than 20 mins.

I want to prevent the race condition here. One way I have in my mind is to use the database set the flag when page is running and remove flag when the job is done. But I really don't want to use database.

Is there any other solution?

The best method (that I've used) involves saving the process id in a "lock file".

function getLock() {
    //First, check to see if the script is already running
    $data = @file_get_contents('path/to/process.pid');
    if ($data && posix_kill($data, 0)) {
        throw new RuntimeException(
            'Unable To Attain Lock, Another Process Is Still Running'
        );
    }
    file_put_contents('path/to/process.pid', posix_getpid());
}

Basically, it reads the file, and then tries to posix_kill the pid contained inside. Note, that when you use a signal of 0 to posix_kill, it merely tells you if the call will succeed (meaning that the process is running).

Doing it this way has the advantage that if the process dies without cleaning up the lock file, it'll be detected and will run again.

You could lock a pre-defined "flag" file using flock() and check whether that file is still locked before starting the script.

Sure, temporary file =]

or fork the process. See more on forking here: http://php.net/manual/en/function.pcntl-fork.php

Suggestion: http://www.saynotoflash.com/archives/multi-process-php-execution/

You could use a marker file, or a shared memory area, or any of the other normal forms of inter-process communication. The gold-plate solution would be to implement a little batch-queue process.

If you make a pid file to indicate a process is running be sure to check when the file was created and have a threshold in place for how long is too long. That way if your job dies before it has an opportunity to delete it's pid file your jobs don't screech to a halt.