Just a curiosity really. I can't find a reason for it anywhere.
Why doesn't php mail() mailto allow you to use a semicolon to separate the email addresses?
So it's for compatibility?
Yes. The RFC cited in PHP documentation explains that the semicolon is used to separate groups of mails, for example:
To: list-a: firstname.lastname@example.org, email@example.com;, list-b: firstname.lastname@example.org, email@example.com;
Maybe that's where I'd seen it used before. Cheers guys.
Some mail clients (Outlook) used to allow semi-colons, but only in the interface, when sending the application has to switch to commas.