Recently I ran into this problem several times – a WordPress Redirect To install.php when the site was already up and running.
In order to resolve this, check the following:
1. MySQL Server Down
I had this problem when I recently tested some free hosts like 000WebHost. The server simply exceeded my resource limit and the MySQL server was offline, giving red error messages even when I logged into phpMyAdmin. The condition was caused by me frequently editing and saving new posts, setting up my home page on MovingInTogether.net.
Confirm that this is the case by giving it some time. Resource limits get reset after certain time and your site should be back online without touching anything. In the meantime I renamed wp-admin/install.php to something else so that it gives a 404 and not cause any damage.
You will need to upgrade your hosting and we recommend SiteGround hosting to all as they are fast, reliable and secure hosting provider across the world.
2. Table Prefix Mismatch In WP-Config
The cause of this issue can also be within wp-config.php file.
I was migrating my site.
I created a new WP installation, uploaded the wp-content files.
Then deleted the tables from the newly installed database (table prefix was ‘wpwc_’) and imported the backed up database (table prefix was ‘wp_’).
This causes WordPress to go to the installer.
You have to double check the $table_prefix field in wp-config.php and make sure that it corresponds to table prefixes in your actual database.
Example: if in the DB your tables look like ”wp_website_options”,
in your wp_config.php it should look like this:
* WordPress Database Table prefix.
* You can have multiple installations in one database if you give each
* a unique prefix. Only numbers, letters, and underscores please!
$table_prefix = ‘wp_website_;
These are the main reasons. If you find another one, leave a comment.