PHPMailer update introduces minor backcompat issue with phpmailerException and SMTP

[Otto42]

[48033] for #41750 replaces the old PHPMailer with the newer version.

In place of the old version, the class-phpmailer.php and class-smtp.php files are deprecated and load the new code, along with adding a class alias.

However, the old class-phpmailer.php file defined both the PHPMailer class, and the phpmailerException class. This means the new one should too.

Recommend adding this to class-phpmailer.php:

require __DIR__ . '/PHPMailer/Exception.php';

and

class_alias( PHPMailer\PHPMailer\Exception::class, 'phpmailerException' );

Similarly, the class-smtp.php is now loading the new file, but it is missing the class alias to account for the namespace. So it should probably have this in it:

class_alias( PHPMailer\PHPMailer\SMTP::class, 'SMTP' );

These aliases would only be needed if these files were loaded, any plugin that relies on them should update to the more direct file includes, and account for the namespaces themselves at that time. The aliases are therefore only for backwards compatibility.

[SergeyBiryukov]

In 48035:

External Libraries: Add class aliases for phpmailerException and SMTP to deprecated files to account for the new namespace.

This ensures backward compatibility with plugins using the old class-phpmailer.php or class-smtp.php files.

Follow-up to [48033].

Props Otto42.
Fixes #50379. See #41750.

[desrosj]

This was detailed in the following dev note: ​https://make.wordpress.org/core/2020/07/01/external-library-updates-in-wordpress-5-5-call-for-testing/