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?

I see.

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: abc@localhost.tld, cba@localhost.tld;, list-b: admin@localhost.tld, info@localhost.tld;

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.