PHP + Oracle + OCIPLogon

I understand that OCIPLogon creates a persitant connection to the Oracle DB, to improve performance... And I must say it does ! However, since OCILogoff is best not used with a persitant logon (and in the latest versions, it doesn't do anything anyway)... How will PHP decide when to cut that connection loose ? We are working on a relatively small website, with 4 users... Within no time, the session limit in Oracle (which is 60) is reached, and the site doesn't work anymore... The website should be able to handle thousands of customers in the future, so it would only take a second to crash... What are we doing wrong? Why aren't the sessions automatically killed by PHP ? Thank you! Gunther