foo() is so short that each thread probably finishes before the next one even gets spawned. If you add a sleep for a random time in foo() before the u++, you may start seeing what you expect.
Straight after the $wpdb->insert() that does the insert, do this: $lastid = $wpdb->insert_id; More information about how to do things the WordPress way can be found in the WordPress codex. The details above were found here on the wpdb class page
In the absence of synchronization this code Object a; public boolean test() { return a != a; } may produce true. This is the bytecode for test() ALOAD 0 GETFIELD test/Test1.a : Ljava/lang/Object; ALOAD 0 GETFIELD test/Test1.a : Ljava/lang/Object; IF_ACMPEQ L1 … as we can see it loads field a to local vars twice, it’s … Read more
A race condition occurs when two or more threads can access shared data and they try to change it at the same time. Because the thread scheduling algorithm can swap between threads at any time, you don’t know the order in which the threads will attempt to access the shared data. Therefore, the result of … Read more