Its the @ symbol in the following line:
$res =get_user($tofriendid,"username",$tofriendemail,"email");
I am assuming the bolded portion is actually "username@domain.com".

Two solutions. Either change the line to this:
$res =get_user($tofriendid,"username",$tofriendemail,'username@domain.com');
(note the quotes) or to this:
$res =get_user($tofriendid,"username",$tofriendemail,"username\@domain.com");

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