Codeigniter: fatal error call to undefined function mysqli_init()

Here's an example of how the error might appear in a Codeigniter application:

Fatal error: Call to undefined function mysqli_init() in /path/to/codeigniter/system/database/drivers/mysqli/mysqli_driver.php on line 135

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This error message is saying that on line 135 of the file mysqli_driver.php, which is located in the directory /path/to/codeigniter/system/database/drivers/mysqli/, the application is trying to call the mysqli_init() function, but that function is not defined. This is likely because the mysqli extension is not installed or enabled. You will need to contact your hosting provider to install it or you can install it manually if you have root access to the server.