mysql pconnect,当它关闭连接时

In genral ,

for mysql_connect have mysql_close ,

but mysql_pconnect does not have any close function ,

i saw in all threads and tutorial , they are saying that for mysql pconnect ,

First, when connecting, the function would first try to find a (persistent)
link that's already open with the same host, username and password.
If one is found, an identifier for it will be returned instead of 
opening a new connection. 

FRO ---> php.net manual

b default there is no close function for mysql_pconnect , then in tutorial,

why they are saying like

the function would first try to find a (persistent) link that's already open with the same host

As per this line :-- may be in some occasion, mysql_pconnect connection will close ,

Then when this connection will close ,in which situation connection goes close mode ,

or is there any time out for cloase connection or time limit ,

i guess this quesiton little but childish ,

The connection will be closed when the web server or FastCGI process dies.