Question

What happens when two different clients call the same php function that have pdo::beginTransaction?

Does one of them fail or can two instances of php execute the contents of a beginTranscation commit block?

IE:

try{
db::beginTransaction();
//queries here
//can two separate php instances go in here at the same time?
db:commit();
}
catch(error e)
{
 db::rollback();
}

Answer 1

Each instance of a PHP script (more accurately, each instance of PDO) opens up a connection to the database (from the DB perspective, a new session). Backend databases (with the exception of a few flat-file ones) support multiple connections, but end up locking their individual resources differently. Depending on the queries executed in your transaction, you may end up causing a deadlock. That said, having multiple connections to the database open at the same time does not necessarily put you in a deadlock scenario.