I'm trying to set up authentication on my server, however, I know little about Php.

I have a php file filled with users and passwords:

 function getPassword( $user )
 {
    // the user/password database. For very small groups
    // of users, you may just get away with expanding this
    // array.
    $passwords= array (
        'user1' => 'password1',
        'user2' => 'password2',
        'user3' => 'password3',
        'user4' => 'password4'
         );
    $password = $passwords[ $user ];
    if ( NULL == $password )
        return NULL;

Without manually editing the array of passwords, I want a php file to read in user inputs for usernames and passwords and append it to the array.

I have a vague idea of how this could work by looking up documentation:

<?php
function fwrite_stream($fp, $string) {
    $fp = fopen('shit.php', 'w');
    for ($written = 0; $written < strlen($string); $written += $fwrite) {
        $fwrite = fwrite($fp, substr($string, $written));
        if ($fwrite === false) {
            return $written;
        }
    }
    return $written;
    fclose($fp);
}
?>

How do I tell this to write to the array?