Nothing, they give the same output.
$adv_email = 'someone@domain.com';
$headers1 = "From: $adv_email";
$headers2 = "From: ".$adv_email;
echo $headers1."<br>".$headers2;
That gives teh output:
From: someone@domain.com
From: someone@domain.com
Are you trying to make it look like it came from a specified account, or actually send it through the account? There is a difference. Sending from the account requires a more robust function set like PEAR::Mail or imap_mail. Masking the FROM header field can be set via headers and can take the form of:
FROM: Persons Name <email_address>
~Brett