Virtualmin configuring the default mailbox for mail() in php?

I'm afraid the connection between "box" for mail() and Virtualmin-Ohm no no. When you call mail(), php is not stupid calls /usr/sbin/sendmail and passes it the headers and the message. Max knows that sendmail is the UID of the process of its caller (so the address is obtained @).
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Another thing is that with headers, you can specify the return address and the From: field of outgoing emails and it is very simple approximately it call mail():
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\rmail('Someone ', 'Subject', 'Message', "From: From whom \Reply-To: Who is responsible ");
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Or you can even specify additional parameters to sendmail via the fifth parameter to mail():
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\rmail('nobody@example.com', 'the subject', 'the message', null, '-fwebmaster@example.com');
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but it's not that flexible for my taste.
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In Google there are tips to change the sendmail_from parameter in php.ini, but they are wrong — this setting only works in Windows.

As written dmiceman, when sending mail with a script - no sender, but you can specify the FROM field.
To do this in the php file.ini specify:
sendmail_path = "/usr/sbin/sendmail -t-i-f mail@domain.tld"
Replacing mail@domain.tld desired address.