In the past, when libmysqlclient could be used, it accepted ipv6 addresses
as hostname without enclosing it first in brackets. However, in mysqlnd
this never worked. In the past this caused a discrepancy between the two
implementations.
Nowadays, mysqli only works with mysqlnd so we don't even have to cater
to libmysqlclient. However, a plain ipv6 address should still work as a
hostname. Also for people migrating to newer PHP versions it's nice if
this keeps working.
The solution is to check if we're dealing with an ipv6 address not yet
enclosed in brackets. In that case we add the brackets automatically.
Closes GH-19750.
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