Undefined constant PHPMailer on line 2017

[Oudalally]

When using PHPMailer 6.8.0 installed manually into my project, sending mail with SMTP is failing with the error
Undefined constant "PHPMailer\PHPMailer\PHPMailer"

This is occuring on line 2017 in PHPMailer.php:

$header = PHPMailer . phpstatic::stripTrailingWSP($header) . static::$LE . static::$LE;.

I have installed PHPMailer as per the instructions for a manual install.
Downgrading to a previous version has worked around the issue.

[Oudalally]

Sorry, this was my mistake.
I had put the sources into a folder called PHPMailer directly, not PHPMailer/src