I have developed site in PHP, If more than 250 users comes to the site at same time, site gets slow how to maintain the site speed.
The answer will depend on why it is slowing down, and to determine that you will need to determine where the bottlenecks are. Trying to fix the PHP will be a waste of time if it turns out your main problem is in the database server, for example.
This is your database problem. Improve your database response time.