I have this doubt, I've searched the web and the answers seem to be diversified. Is it better to use mysql_pconnect over mysql_connect when connecting to a database via PHP? I read that pconnect scales much better, but on the other hand, being a persistent connection... having 10 000 connections at the same time, all persistent, doesn't seem scalable to me.
Thanks in advance.
Persistent connections should be unnecessary for MySQL. In other databases (such as Oracle), making a connection is expensive and time-consuming, so if you can re-use a connection it's a big win. But those brands of database offer connection pooling, which solves the problem in a better way.
Making a connection to a MySQL database is quick compared to those other brands, so using persistent connections gives proportionally less benefit for MySQL than it would for another brand of database.
Persistent connections have a downside too. The database server allocates resources to each connection, whether the connections are needed or not. So you see a lot of wasted resources for no purpose if the connections are idle. I don't know if you'll reach 10,000 idle connections, but even a couple of hundred is costly.
Connections have state, and it would be inappropriate for a PHP request to "inherit" information from a session previously used by another PHP request. For example, temporary tables and user variables are normally cleaned up as a connection closes, but not if you use persistent connections. Likewise session-based settings like character set and collation. Also, LAST_INSERT_ID() would report the id last generated during the session -- even if that was during a prior PHP request.
For MySQL at least, the downside of persistent connections probably outweighs their benefits. And there are other, better techniques to achieve high scalability.
Update March 2014:
MySQL connection speed was always low compared to other brands of RDBMS, but it's getting even better.
See http://mysqlserverteam.com/improving-connectdisconnect-performance/
In MySQL 5.6 we started working on optimizing the code handling connects and disconnects. And this work has accelerated in MySQL 5.7. In this blog post I will first show the results we have achieved and then describe what we have done to get them.
Read the blog for more details and speed comparisons.