How to change variables in the .env file dynamically in Laravel?

I want to create a Laravel web app that allows an admin user to change some variables(such as database credentials) in the .env file using the web backend system. But how do I save the changes?

There is no built in way to do that. If you really want to change the contents of the .env file, you'll have to use some kind of string replace in combination with PHP's file writing methods. For some inspiration, you should take a look at the key:generate command: KeyGenerateCommand.php:

$path = base_path('.env');

if (file_exists($path)) {
    file_put_contents($path, str_replace(
        'APP_KEY='.$this->laravel['config']['app.key'], 'APP_KEY='.$key, file_get_contents($path)
    ));
}

After the file path is built and the existence is checked, the command simply replaces APP_KEY=[current app key] with APP_KEY=[new app key]. You should be able to do the same string replacement with other variables.
Last but not least I just wanted to say that it might isn't the best idea to let users change the .env file. For most custom settings I would recommend storing them in the database, however this is obviously a problem if the setting itself is necessary to connect to the database.

Yet another implementation, in case you have something like:

A = B #this is a valid entry

In your .env file

public function updateEnv($data = array())
{
    if (!count($data)) {
        return;
    }

    $pattern = '/([^\=]*)\=[^\n]*/';

    $envFile = base_path() . '/.env';
    $lines = file($envFile);
    $newLines = [];
    foreach ($lines as $line) {
        preg_match($pattern, $line, $matches);

        if (!count($matches)) {
            $newLines[] = $line;
            continue;
        }

        if (!key_exists(trim($matches[1]), $data)) {
            $newLines[] = $line;
            continue;
        }

        $line = trim($matches[1]) . "={$data[trim($matches[1])]}\n";
        $newLines[] = $line;
    }

    $newContent = implode('', $newLines);
    file_put_contents($envFile, $newContent);
}

Update Erick's answer with consideration of $old values covering sting, bool and null values.

public static function changeEnvironmentVariable($key,$value)
{
    $path = base_path('.env');

    if(is_bool(env($key)))
    {
        $old = env($key)? 'true' : 'false';
    }
    elseif(env($key)===null){
        $old = 'null';
    }
    else{
        $old = env($key);
    }

    if (file_exists($path)) {
        file_put_contents($path, str_replace(
            "$key=".$old, "$key=".$value, file_get_contents($path)
        ));
    }
}

To extend on lukasgeiter's and other's answer above, using regex to match the .env would be better, , as unlike app.key, the variable to put in .env may not be in the config.

Below is the code I used when experimenting on custom artisan command. This code generates a key for XChaCha encryption (XCHACHA_KEY=?????):

$path = base_path('.env');
if (file_exists($path)) {
    //Try to read the current content of .env
    $current = file_get_contents($path);   

    //Store the key
    $original = [];
    if (preg_match('/^XCHACHA_KEY=(.+)$/m', $current, $original)) { 
    //Write the original key to console
        $this->info("Original XChaCha key: $original[0]"); 

    //Overwrite with new key
        $current = preg_replace('/^XCHACHA_KEY=.+$/m', "XCHACHA_KEY=$b64", $current);

    } else {
    //Append the key to the end of file
        $current .= PHP_EOL."XCHACHA_KEY=$b64";
    }
    file_put_contents($path, $current);
}
$this->info('Successfully generated new key for XChaCha');

Using preg_match() allows the original key to be retrieved as needed, also allows the key to be changed even if the actual value is not known.