MySQL close connection call in famous CMSs?

Hello,
i listen it is goodif php script instrct to close mysql connection after its done.
Is this practice still benefficial anyhow to limit paralel connections on server?
If so, please do have common CMSs all includes this function?
WordPress
Joomla
vBulletin
PHPBB
if no, what i can add into the scripts while im noob in php, mysql?
Thank you

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