What is thread safe or non-thread safe in PHP?

In PHP, a thread-safe version of PHP is built with additional constructs to ensure that certain actions are atomic, which means that they are performed in their entirety without any other thread interfering. This is important in a multi-threaded environment because it allows you to write code that relies on shared states without worrying about race conditions or other synchronization issues.

On the other hand, a non-thread-safe version of PHP does not have these additional constructs and, therefore, cannot guarantee that certain actions will be atomic. This means it is unsuitable for use in a multi-threaded environment because multiple threads can interfere with each other's execution.

When choosing which PHP version to use, it is important to consider whether your code will run in a multi-threaded environment. If it will, then you should use a thread-safe version of PHP. If it will not, you can use either the thread-safe or non-thread-safe version.