How to prevent the cron job execution, if it is already running

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Advisory locking is made for exactly this purpose.

You can accomplish advisory locking with flock(). Simply apply the function to a previously opened lock file to determine if another script has a lock on it.

$f = fopen('lock', 'w') or die ('Cannot create lock file');
if (flock($f, LOCK_EX | LOCK_NB)) {
    // yay
}

In this case I’m adding LOCK_NB to prevent the next script from waiting until the first has finished. Since you’re using cron there will always be a next script.

If the current script prematurely terminates, any file locks will get released by the OS.

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