多个PHP脚本可以同时运行吗？

I am building a turn-based multiplayer game with Flash and PHP. Sometimes two users may call on the same PHP script at the same time. This script is designed to write some information to the database. But that script should not run if that information has already been written by another user, or else the game will break. If PHP processes these scripts sequentially (similar to how MySQL queues up multiple queries), then only one script should run in total and everything should be fine.

However, I find that around 10% of the time, BOTH user's scripts are executed. My theory is that the server sometimes receives both user requests to run the script at exactly the same time and they both run because neither detected that anything has been written to the database yet. Is it possible that both scripts were executed at the same time? If so, what are the possible solutions to this problem.

That's very plausible that they do. Look into database transactions.

Briefly, database transactions are used to control concurrency between programs that access the database at the same time. You start a transaction, then execute multiple queries and finally commit the transaction. If two scripts overlap each other, one of them will fail.

Note that isolation levels can further give fine grained control of how much the two (or more) competing scripts may share. Typically all are allowed to ready from the database, but only one is allowed to write. So the error will happen at the final commit. This is fine as long as all side effects are happening in the database, but not sufficient if you have external side effects (Such as deleting a file or sending an email). In these cases you may want to lock a table or row for the duration of the transaction or set the isolation level.

It doesnt matter, if it is PHP, C, Java what ever. At the same time can run max only as much processes, as you have CPUs (and cores). There can be running lets say 100 processes at the same time, if you have only 2 cores. Only 2 are running, rest is waiting.

Now it depends what you see under run. If you take it as active or if you take also waiting processes. Secondly, it depends on your system configuration, how many processes can wait and on your system specs.

THis is indeed possible. You can try locking and unlocking tables at the beginning and end of your scripts.

Though this will slow down some requests, as they would have to first wait for the locked tables to be unlocked.

Here is an example of SQL table locking that you can use so that the first PHP thread which grabs the DB first will lock the table (using lock name "awesome_lock_row") until it finally releases it. The second thread attempts to use the same table, and since the lock name is also "awesome_lock_row"), it keeps on waiting until the first PHP thread has unlocked the table.

For this example, you can try running the same script perhaps 100 times concurrently as a cron job and you should see "update_this_data" number field increment to 100. If the table hadn't been locked, all the concurrent 100 threads would probably first see "update_this_data" as 0 at the same time and the end result would have been just 1 instead of 100.

<?php
$db = new mysqli( 'host', 'username', 'password', 'dbname'); 

// Lock the table
$db->query( "DO GET_LOCK('awesome_lock_row', 30)" ); // Timeout 30 seconds

$result = $db->query( "SELECT * FROM table_name" );
if($result) {
    if ( $row = $result->fetch_object() )
        $output = $row;
    $result->close();  
}

$update_id = $output->some_id;

$db->query( UPDATE table_name SET update_this_data=update_this_data+1 WHERE id={$update_id} );

// Unlock the table
$db->query( "DO RELEASE_LOCK('awesome_lock_row')" ); 
?>

Hope this helps.