如何在PHP中的文本文件中编辑不同用户的特定行

Question

I have a Subversion server running with AuthzSVNAccessFile specified in a virtual host. The file "permissions.txt" should be edited by the users themselves to add or remove team members but limited to max. 4 members. Possible via PHP and how to implement it? In PHP each logged in user has the $username = user_x; whereas x is an integer.

The AuthzSVNAccessFile "permissions.txt":

[repo_1001:/]
user_1 = rw
# the lines below only editable by user_1
user_1-member_1 = r
user_1-member_2 = r
user_1-member_3 = r
user_1-member_4 = r

[repo_1002:/]
user_2 = rw
# the lines below only editable by user_2
user_2-member_1 = rw
user_2-member_2 = rw

Reading and writing the file in PHP:

$file_permissions = "/.../permissions.txt";

if (isset($_POST) && ...) {
    $textFileContents = $_POST['textFileContents'];
    $fd=fopen($file_permissions ,"w");
    fwrite($fd, $textFileContents);
    fclose($fd);
}

$fd = fopen($file_permissions,"r");
$textFileContents = fread($fd,filesize($file_permissions));
fclose($fd);

var_dump($textFileContents);

I tried "AbraCadaver" mentioned parse-ini-file function but I do not get any output.
Even no errors are seen:

$file_permissions = "/.../permissions.txt";

$textFileContents = parse_ini_file($file_permissions);

var_dump($textFileContents);

The output is just "NULL".

ADD: The cause why I got "NULL" is that my php.ini on the remote server was configured to disable the function parse_ini_file for security reasons.
Now it works!