Can multiple PHP scripts run at the same time?

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.

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.

Sounds, at first glance, like what keeps a 2nd instance of the script to roll just does not happen fast enough, 10% of the time... I understand that you already have some kind of a 'lock' like someone told you to add, which is great; as someone mentioned above, always put this lock FIRST THING in your script, if not even before calling the script (a.k.a in parent script). Same goes for competing functions / objects etc...

Just a note though, I was directed here by google and what i wanted to find out is if script B will run IN AN IFRAME (so in a 'different window' if you wish) if script A is not finished running; basically your title is a bit blurry. Thank you very much.

Fortunately enough we're in the same pants : I'm programing an Hearthstone-like card game using php (which I know, ain't suited for this at all, but I just like challenging tasks, (and okay, that's the only language i'm familiar with)). Basically I gotta keep multiple 'instants' or actions if you prefer from triggering while another set of global event/instant - instants - sub-instants is rolling. This includes NEVER calling a function that has an event into it into the same rolling snipet, EXCEPT if I roll a while on a $_SESSION variable with value y that only does sleep(1) (that happens in scritpt A); while $_SESSION["phase"] == "EndOfTurnEffects" and then continue to roll until $_SESSION["phase"] == "StandBy" (other player's turn), and I wish script B to mofity $_SESSION["phase"]. Basically if script B does not run before script A is done executing, I'm caught in an endless loop of the while statement...

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.

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.