Wordpress redirect issue, headers already sent - wordpress

Question

I am wondering, based on the code bellow, where I would want to put my wp_redirect function because where it currently is does nothing but spazzes out and sais:
Warning: Cannot modify header information - headers already sent by (output started at /***/***/WordPress/WordPressDev/wp-includes/script-loader.php:664) in /***/***/WordPress/WordPressDev/wp-includes/pluggable.php on line 881
Which I get because the page has already loaded. but I am un sure where to call this function.
I have replace my web site and any "personal data" with stars and example.com. How ever this code does work, it just wont redirect me.
thoughts?
function get_latest_version_zip(){
global $wp_filesystem;
if(current_user_can('update_themes')){
$aisis_file_system_structure = WP_Filesystem();
$aisis_cred_url = 'admin.php?page=aisis-core-update';
if($aisis_file_system_structure == false){
request_filesystem_credentials($aisis_cred_url);
$this->credential_check = true;
}
$aisis_temp_file_download = download_url( 'http://example.com/aisis/aisis_update/Aisis2.zip' );
if(is_wp_error($aisis_temp_file_download)){
$error = $aisis_temp_file_download->get_error_code();
if($error == 'http_no_url') {
add_action( 'admin_notices', 'aisis_framework_download_update_erors' );
}
}
$aisis_unzip_to = $wp_filesystem->wp_content_dir() . "/themes/" . get_option('template');
$this->delete_contents_check(); //Check if we need to delete the aisis core folder.
$aisis_do_unzip = unzip_file($aisis_temp_file_download, $aisis_unzip_to);
unlink($aisis_temp_file_download); //delete temp jazz
if(is_wp_error($aisis_do_unzip)){
$error = $aisis_do_unzip->get_error_code();
if($error == 'incompatible_archive') {
$this->aisis_incompatible_archive_errors();
}
if($error == 'empty_archive') {
$this->aisis_empty_archive_errors();
}
if($error == 'mkdir_failed') {
$this->aisis_mkdir_failed_errors();
}
if($error == 'copy_failed') {
$this->aisis_copy_failed_errors();
}
return;
}
//throwing errors
wp_redirect(admin_url('admin.php?page=aisis-core-options'));
exit;
}
}
in my functions.php file I placed the following code:
function callback($buffer){
return $buffer;
}
function add_ob_start(){
ob_start("callback");
}
function flush_ob_end(){
ob_end_flush();
}
add_action('wp_head', 'add_ob_start');
add_action('wp_footer', 'flush_ob_end');
with this I still get the error, I think I misunderstanding something....

Answer 1

Just replace the following line
add_action('wp_head', 'add_ob_start');
with
add_action('init', 'add_ob_start');
Output buffering should start before anything sent/echoed to the browser and wp_head hook occurs a bit later than init hook and till then headers already sent and also Keep/place it at the top of your functions.php before anything echoed/sent to the browser.

Answer 2

The problem is that somewhere in wordpress the header() function has been called and some output was already sent to the client while output buffering is off.
Headers have to be sent before any output, otherwise you get the error you described.
wp_redirect(admin_url('admin.php?page=aisis-core-options'));
The above line sets a header like this: header('Location: admin.php......');
Turning on output buffering via php.ini, at the index.php of wordpress or simply before anything is echo'ed to the client should take care of the error.
Details/Documentation can be found here: http://php.net/manual/en/book.outcontrol.php
simplest way i can think of is make your wordpress index.php look like this:
ob_start();
// content of your index.php here
ob_flush();

Answer 3

Another possibility would be adding a priotity:
add_action('wp_head', 'add_ob_start', 1);
The third param is $priority.
Also, if you're hooking in more than one function, it gives you complete control of the execution chain.