PHP Fatal error: Cannot declare class PHPMailer\PHPMailer\PHPMailer

[david.binda]

With PHPMailer living in a new directory, including the wp-includes/class-phpmailer.php from code may result in a fatal error, ie.: when running the phpunit.

The change in https://core.trac.wordpress.org/changeset/48033/#file7 requires once the PHPMailer class directly, so the class gets declared. In case there is a plugin requiring the wp-includes/class-phpmailer.php (even when require_once is used), the file requires the file declaring the PHPMailer class again ( see https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-phpmailer.php?rev=48035#L15 ).

I believe that require_once in wp-includes/class-phpmailer.php should be used in order to prevent mentioned fatal errors.

[SergeyBiryukov]

In 48530:

Mail: Make sure the PHPMailer class is only required once if a plugin requires wp-includes/class-phpmailer.php directly.

Follow-up to [48033].

Props david.binda.
Fixes #50716. See #41750.

[SergeyBiryukov]

In 50022:

Mail: Make sure the SMTP class is only required once if a plugin requires wp-includes/class-smtp.php directly.

Follow-up to [48033], [48530].

Props oellin, greatsaltlake, audrasjb.
Fixes #52369. See #50716, #41750.