How to get form input results written to a file?

On one of my websites, I use the html "form input" code shown below
for visitors to input their email address to be emailed to me. This
works fine.

What would I need add to this code to get the same submitted
information written to a file in my web hosted space on my web
host's server in addition to having the information sent to me by
email?

<form action=http://www.bluehost.com/bluemail method=POST>
 <input type=hidden name=sendtoemail value=joe@vsiq.com>
 Please enter your email address:<p>
 <input type=text name=mailfrom size=30 maxlength=70>
 <input type=submit value=Send>
</form>



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