add code automatically in htaccess or any file when activate the plugin

function myplugin_enable_flush_rules() {

//add default value for notification
if (get_option('magNotifiy') == 0 ) {
update_option( 'magNotifiy', '1', '', 'yes' );
}else{
add_option( 'magNotifiy', '1', '', 'yes' );
}

//add code in .htaccess file for enable HTTP_AUTHORIZATION
$WPHE_new_content = '

RewriteEngine on
RewriteCond %{HTTP:Authorization} ^(.*)
RewriteRule ^(.*) - [E=HTTP_AUTHORIZATION:%1]
SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1';
$WPHE_orig_path = ABSPATH.'.htaccess';

@clearstatcache();
@chmod($WPHE_orig_path, 0666);

$myfile = fopen($WPHE_orig_path , "r") or die("Unable to open file!");
$htaccess_exist =  fread($myfile,filesize($WPHE_orig_path));
fclose($myfile);

if( strpos( $htaccess_exist, 'HTTP_AUTHORIZATION' ) !== false ) {
   //echo "already added code";
}else{
if (!is_writable($WPHE_orig_path) ) {
echo ".htaccess file not writable permission.";  exit();
}
$WPHE_new_content = str_replace('\\\\', '\\', $WPHE_new_content);
$WPHE_new_content = str_replace('\"', '"', $WPHE_new_content);
$WPHE_write_success = @file_put_contents($WPHE_orig_path, $WPHE_new_content, FILE_APPEND |LOCK_EX);
@clearstatcache();
}
}
// on plugin activation, call the function that will make flush_rules
register_activation_hook( __FILE__, 'myplugin_enable_flush_rules' );