PHP PDO(PHP Data Object) allows you to access database including PostgreSQL within PHP scripts. Good thing with PDO is, you do not need to heavily modify your PHP script even if you change the database product you want to use. This resembles to JDBC driver or Perl DBI. Moreover you could use database product dependent interface. Which means you can enjoy the full power of PostgreSQL with sacrificing portability of course. There's no free lunch. In summary, PHP PDO is great and I would like to recommend to anyone who is thinking about serious database programming in PHP.
One day I came across a complaint from pgpool-II user. According to him, his PHP client was blocked by pgpool-II while trying to connect to it. Ok, this is a typical situation when user misconfigured num_init_children and I thought he tried to connect pgpool-II concurrently with more than num_initi_children sessions. But his answer is, no. I suspected the PDO internal but I was too lazy to look into PDO source code. Instead I use strace to see what PDO is doing. To make the long story short, PDO releases the connection to pgpool-II long after "$dbh = null;"($dbh is the "handle" of the connection. This should release the connection). So if his script continues and connects to pgoool-II again after he thinks that he releases the connection, the script actually has two concurrent connections to pgpool-II. Of course this will create more connections than he thought.
I think the reason why many people do not come up with similar complaint is, their script usually create just one connection and release it at the end of the script.