如何使用PHP更新用户名和密码的文件

I use a public domain url shortener called yourls which I have made changes to. The php script uses a config file that users passwords and logins are stored in which is config.php

The format for this file is various settings etc and then a section for the passwords and level of the user. Here is the section within the file I need to work with.

 $yourls_user_passwords = array(
      'guest' =>    'phpass:!2a!08!evrztH.lysEouhMAai50MO1sl4/C234Q99HoL3jezyfZU4gMp95SG' /*  Password encrypted by YOURLS */ ,
      'chuckb' =>   'phpass:!2a!08!q53CY5Xp6rY7Nx65Jg98H2q9trxb8GYpAcz7in06b3Qk4G8vYrZ6' /* Password  encrypted by YOURLS */ ,
      'jimc' => 'phpass:!2a!08!q74CErXp6rY7Hx68egr8t2q9trxb9FYpFcz9in06b6Tk5e8vGrZ3' /* Password encrypted by YOURLS */ ,
// You can have one or more 'login'=>'password' lines
     );
 //** assign user role */

 $authmgr_role_assignment = array(
 'administrator' => array(
   'chuckb',

  ),
 'editor' => array(
   'jimc',

  ),
   'contributor' => array(
    'guest',    
   ),

   );

I would like to know how to using a php form add, new users to this list and delete existing ?

I also would like if possible the file to be readonly until this scrip updates it and then have it be made read only again if that is possible ?

I have never worked with php writing to a file. any help would be appreciated.