I don't understand this piece of PHP connect database code

I started learning PHP a few days ago and I am currently learning rest api. I tried asking gpt about it but I could not understand it.

<?php function adatokLekerdezese($muvelet ){ $db = new mysqli("localhost", "root", "" , "autok"); if($db->connect_errno !=0)return $db-> connect_error; $eredmeny = $db->query($muvelet); if ($db -> errno != 0){ $db ->close(); return $db-> error; } $adatok = ($eredmeny-> num_rows > 0) ? $eredmeny-> fetch_all(MYSQLI_ASSOC) : []; $db->close(); return $adatok; } function adatokValtoztatasa($muvelet){ $db = new mysqli("localhost", "root", "", "autok"); if($db->connect_errno !=0)return $db -> connect_error; $db -> query($muvelet); if($db->errno !=0){ $db ->close(); return $db -> error; } return $db -> affected_rows > 0 ? true : false; } ?> include "./sql_fuggveny.php"; $metodus = $_SERVER["REQUEST_METHOD"]; $uri = parse_url($_SERVER["REQUEST_URI"], PHP_URL_PATH); $uri = explode("/", $uri); $bodyAdatok = json_decode(file_get_contents("php://input"), true);