Description:
------------
The docs for the mail() function state the first parameter ($to) as being obligatory. They also state that "the formatting of this string must comply with ? RFC 2822".
This would lead me to expect that passing a string not containing a valid address would cause a warning or at least a notice. This isn't so.
While technically I'm fairly sure this is correct behaviour (AFAICT RFC 2822 states that either To, Cc or Bcc should be included, so leaving *out* To shouldn't be a problem and in fact isn't, and non-email addresses are also supported) the docs suggest that under PHP it is in fact illegal.
I would expect that passing NULL or an empty string would result in the entire To: field being omitted in the headers (it's now in fact empty, which doesn't seem to conform to RFC 2822 which says that if you *do* include any such header it should contain a comma-separated list of at *least* 1 address). Whether or not a notice should follow something that doesn't appear to be an emailaddress is a matter for debate - I'd personally leave regexing to the users of the function, but then the manual isn't correct.

So, actually it's something of a bug/feature in the mail() function as well I believe, and passing NULL as the first parameter should cause the To-header to be omitted.
But since including a To-header seems like good form mostly the docs are, I feel, somewhat incomplete in this respect.

Reproduce code:
---------------
<?php

mail(
        NULL,
        'test',
        NULL,
        <<<EOT
Bcc: marijn@monomelodies.nl

test

EOT
);

Expected result:
----------------
Depending on how you feel about $to being obligatory, either a better-formed email being sent, or a notice or even warning thrown by PHP.

Actual result:
--------------
[...snip...]
To:           <-- violates RFC 2822
Subject: test

test