This is the reply from my customer on the subject
Thank you for trying, but unfortunately it didn’t work. I changed all instances of split( to explode( but it’s made no difference. Other functions that included the word split I left alone, such as
$dotted = preg_split( "/[.]+/", $dotted);
$encoded = chunk_split(base64_encode($str), 76, $this->LE);
$splitBreak = "--WAMULTIBREAKWA";
And I guess that must be right.
I even changed return document.cookie.match(re).split("=") //return its value, even though it was in a file that had nothing to do with mailing.
All is back as-was now, and running brilliantly in 5.2.