ขอคำแนะนำ การเขียน PHP กับ SQL หน่อยครับ เกียวกับ การเขียน PHP ต่อกับ MYSQL

<?php
    if(function_exists("protect_page") === false) {
        function protect_page() {
            if(logged_in() === false) {
                header("Location: login.php");
                exit();
            }
        }
    }
    if(function_exists("logged_in_redirect") === false) {
        function logged_in_redirect() {
            if(logged_in() === true) {
                header("Location: index.php");
                exit();
            }
        }
    }
    if(function_exists("logged_in") === false) {
        function logged_in() {
            return (isset($_SESSION['username'])) ? true : false;
        }
    }
    if(function_exists("rand_str") === false) {
        function rand_str($length = 10) {    
            return substr(str_shuffle("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"), 0, $length);
        }
    }
    if(function_exists("array_sanitize") === false) {
        function array_sanitize(&$item) {
            $item = mysqli_real_escape_string($mysqli_db,$item);
        }
    }
    
    if(function_exists("user_exists") === false) {
        function user_exists($username) {
            if(file_exists(USER_FILE_PATH . $username . ".yml") === true) {
                return true;
            } else {
                return false;
            }
        }
    }
    if(function_exists("item_data") === false) {
        function item_data($item_id) {
            global $mysqli_db;
            $data = array();
            $item_id = (int)$item_id;
            
            $func_num_args = func_num_args();
            $func_get_args = func_get_args();
            
            if($func_num_args > 1) {
                unset($func_get_args[0]);
                $fields = "`" . implode("`, `", $func_get_args) . "`";
                $result = mysqli_query($mysqli_db, "SELECT ".$fields." FROM `items` WHERE `item_id` = ".sanitize($item_id)."");
                $data = mysqli_fetch_assoc($result);
                return $data;
            }
        }
    }
    if(function_exists("user_money") === false) {
        function user_money($username) {
            $data = spyc_load_file(USER_FILE_PATH . $username . ".yml");
            if(isset($data['money']) === true && empty($data['money']) === false) {
                return $data['money'];
            } else {
                return "0.0";
            }
        }
    }
    if(function_exists("user_data") === false) {
        function user_data($username) {
            $data = spyc_load_file(USER_FILE_PATH . $username . ".yml");
            if(isset($data['money']) === true && empty($data['money']) === false) {
                unset($data['money']);
            }
            return $data;
        }
    }
    if(function_exists("sanitize") === false) {
        function sanitize($data) {
            global $mysqli_db;
            return mysqli_real_escape_string($mysqli_db, $data);
        }
    }
    if(function_exists("mysqli_result") === false) {
        function mysqli_result($res, $row, $field=0) { 
            $res->data_seek($row); 
            $datarow = $res->fetch_array(); 
            return $datarow[$field]; 
        } 
    }
    if(function_exists("ago") === false) {
        function ago($date) {
            if(empty($date)) {
                return "No date provided";
            }
         
            $periods         = array("second", "minute", "hour", "day", "week", "month", "year");
            $lengths         = array("60","60","24","7","4.35","12");
         
            $now             = time();
            $unix_date       = $date;
         
               // check validity of date
            if(empty($unix_date)) {    
                return "Bad date";
            }
         
            // is it future date or past date
            if($now > $unix_date) {    
                $difference     = $now - $unix_date;
                $tense         = "ago";
         
            } else {
                $difference     = $unix_date - $now;
                $tense         = "from now";
            }
         
            for($j = 0; $difference >= $lengths[$j] && $j < count($lengths)-1; $j++) {
                $difference /= $lengths[$j];
            }
         
            $difference = round($difference);
         
            if($difference != 1) {
                $periods[$j].= "s";
            }
         
            return "$difference $periods[$j] {$tense}";
        }
    }
?>