Slim Application Error

The application could not run because of the following error:

Details

Code: 2
Message: Cannot modify header information - headers already sent by (output started at /home2/presid42/public_html/site.php:521)
File: /home2/presid42/public_html/vendor/bergasoftware/php-classes/src/Model/User.php
Line: 83

Trace

#0 [internal function]: Slim\Slim::handleErrors(2, 'Cannot modify h...', '/home2/presid42...', 83, Array)
#1 /home2/presid42/public_html/vendor/bergasoftware/php-classes/src/Model/User.php(83): header('Location: /admi...')
#2 /home2/presid42/public_html/admin.php(7): Bergasoftware\Model\User::verifyLogin()
#3 [internal function]: {closure}()
#4 /home2/presid42/public_html/vendor/slim/slim/Slim/Router.php(200): call_user_func_array(Object(Closure), Array)
#5 /home2/presid42/public_html/vendor/slim/slim/Slim/Slim.php(1210): Slim\Router->dispatch(Object(Slim\Route))
#6 /home2/presid42/public_html/vendor/slim/slim/Slim/Middleware/Flash.php(86): Slim\Slim->call()
#7 /home2/presid42/public_html/vendor/slim/slim/Slim/Middleware/MethodOverride.php(94): Slim\Middleware\Flash->call()
#8 /home2/presid42/public_html/vendor/slim/slim/Slim/Middleware/PrettyExceptions.php(67): Slim\Middleware\MethodOverride->call()
#9 /home2/presid42/public_html/vendor/slim/slim/Slim/Slim.php(1159): Slim\Middleware\PrettyExceptions->call()
#10 /home2/presid42/public_html/index.php(33): Slim\Slim->run()
#11 {main}