Msg system with txt file

now here comes my hard part i trying to make a msg system and i want a user to be able to write to any txt file they want
im using something like this for my messageboard but i need it so the users can type in what file they wnt to write it now it writeing to mail.txt the user they will message to will have there own txt file like if you would sign up it will create like php-forums.txt
and if someone wanted to messge you they would type in php-fourms and it will send there info to php-fourms.txt
this is the code i would need chaged dotn know how to do this can anyone help
this is the pice of the code i think needs to be changed

but here is the whole code
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try http://www.php.net and type in their search field writing to files and make sure you select in the drop down box next to it Online Documentation.

Auron