Obtain exclusive read/write lock on a file for atomic updates

I want to have a PHP file that is used as a counter. It will a) echo the current value of a txt file, and b) increment that file using an exclusive lock so no other scripts can read or write to it while it's being used.

User A will write and increment this number, while User B requests to read the file. Is it possible that User A can lock this file so no one can read or write to it until User A's write is finished?

I've used flock in the past, but I'm not sure how to get the file to wait until it is available, rather than quitting if it's already been locked

My goal is:

LOCK counter.txt; write to counter.txt;

while at the same time

Read counter.txt; realize it's locked so wait until that lock is finished.

//

$fp = fopen("counter.txt", 'w+');
if(flock($fp, LOCK_EX)) {
    fwrite($fp, $counter + 1);
    flock($fp, LOCK_UN);
} else {
    // try again??
}

fclose($fp);