What consequences can occur when multiple transactions are executed simultaneously in a system?

When multiple transactions are executed simultaneously in a system, the following consequences may occur:

• Race conditions: when two transactions attempt to modify the same data at the same time, which can lead to inconsistency.
• Deadlocks: a situation where two or more transactions block each other and cannot continue execution.
• Lost updates: when changes made by one transaction are overwritten by another without acknowledgment.
• Dirty reads: when one transaction reads data that has been modified but not yet committed by another transaction.

To prevent these issues, mechanisms such as transaction isolation, locking, and isolation levels (e.g., READ COMMITTED, SERIALIZABLE) are used. In PHP, it is important to properly use transactions and control their isolation when working with databases.