Use MySQLi when available by default

[dd32]

4 years ago when we added MySQLi support to WordPress #21663 we gated MySQLi support on the PHP version, PHP 5.5+ (or if they're running a development version)

MySQLi has been extremely stable over the last 4 years, and I think it's time we open it up to more users.
For users on PHP 5.2~5.4 switching to MySQLi doesn't provide any benefits, but it does allow WordPress to consider using more of the MySQLi features (for example, #42352) as more sites change over in the future.

The attached patch enables MySQLi by default.
Setting to Future pending agreement.

[pento]

Let's do it. We can roll this out in a 4.9.x release.

[dd32]

I'm hesitant to want to roll it out in a point release, purely for fear of breakage.

That being said, I can't imagine anything would really break, as we include a fallback of MySQLi -> MySQL in the event of a connection failure.. so.. lets go for it?

[pento]

Doing enhancements in point releases is going to take a bit of getting used to. Best way is to dive right in!

[dd32]

In 42388:

WPDB: Always use mysqli when available.

This change makes WordPress use mysqli on PHP <5.5 when available, instead of only in PHP 5.5+ and development installs.

WPDB includes a fallback to mysql in the event that the database connection fails with mysqli so incompatibilities should be few and far between.

Fixes #42812 for trunk.

[SergeyBiryukov]

In 42455:

WPDB: Always use mysqli when available.

This change makes WordPress use mysqli on PHP <5.5 when available, instead of only in PHP 5.5+ and development installs.

WPDB includes a fallback to mysql in the event that the database connection fails with mysqli so incompatibilities should be few and far between.

Props dd32.
Merges [42388] to the 4.9 branch.
Fixes #42812.

[rhirschfeld]

It was perhaps a mistake to roll this out in a point release because:

• Those using mysql on sites where both mysql and mysqli are available experienced breakage (I can't imagine I'm the only one).
• It's not clear to the naive user that the breakage can be corrected by defining WP_USE_EXT_MYSQL as true (or even what caused the breakage), and until it is corrected database corruption can ensue.
• Users have no opportunity to make a backup before an automatic update.

[maizey]

I just spent the last 4 hours trying to figure out what caused many of my websites to break and fixing it. Many plugins rely on the open MySQL-Connection and cannot detect that suddenly that was switched to MySQLi. I also used the open MySQL-Connection for many things.

Thanks to @rhirschfeld who probably saved me another 4 hours with the hint on defining WP_USE_EXT_MYSQL.

(Yes i created an account for this. This issue is big.)