แนวทางตรวจสอบ error การเชื่อมต่อ และการคิวรี่ข้อมูลจาก database

<?php  
$mysqli = new mysqli("localhost", "root","","test");  
/* check connection */
if ($mysqli->connect_errno) {  
    printf("Connect failed: %s\n", $mysqli->connect_error);  
    exit();  
}  
if(!$mysqli->set_charset("utf8")) {  
    printf("Error loading character set utf8: %s\n", $mysqli->error);  
    exit();  
}

<?php  
require_once("dbconnect.php");

display_errors = on

<?php
require_once("dbconnect_no_file.php");

Warning: require_once(dbconnect_no_file.php): failed to open stream: No such file or directory in C:\xampp\htdocs\demo\test_error.php on line 2

Fatal error: require_once(): Failed opening required 'dbconnect_no_file.php' (include_path='C:\xampp\php\PEAR') in C:\xampp\htdocs\demo\test_error.php on line 2

if ($mysqli->connect_errno) {  
    printf("Connect failed: %s\n", $mysqli->connect_error);  
    exit();  
}  

if ($mysqli->connect_errno) {  
    printf("Connect failed: %s\n";  
    exit();  
}  

php_flag display_startup_errors on
php_flag display_errors on

/** Error reporting */
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);

/** Error reporting */
error_reporting(0);
ini_set('display_errors', FALSE);
ini_set('display_startup_errors', FALSE);

<?php
/** Error reporting */
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);
require_once("dbconnect.php");

<?php
/** Error reporting */
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);
/*error_reporting(0);
ini_set('display_errors', FALSE);
ini_set('display_startup_errors', FALSE);*/

<?php
include_once("report_error.php");
require_once("dbconnect.php");

Warning: mysqli::__construct(): (HY000/1049): Unknown database 'test1' in C:\xampp\htdocs\demo\dbconnect.php on line 2
Connect failed: Unknown database 'test1'

Warning: mysqli::__construct(): (HY000/1045): Access denied for user 'root'@'localhost' (using password: YES) in C:\xampp\htdocs\demo\dbconnect.php on line 2
Connect failed: Access denied for user 'root'@'localhost' (using password: YES)

<?php
include_once("report_error.php");
require_once("dbconnect.php");

$sql = "
SELECT * FROM tbl_doc
";
$result = $mysqli->query($sql2);
$row = $result->fetch_assoc();
echo $row['user_v5'];
?>

Notice: Undefined variable: sql2 in C:\xampp\htdocs\demo\test_error.php on line 8

Warning: mysqli::query(): Empty query in C:\xampp\htdocs\demo\test_error.php on line 8

Fatal error: Uncaught Error: Call to a member function fetch_assoc() on boolean in C:\xampp\htdocs\demo\test_error.php:9 Stack trace: #0 {main} thrown in C:\xampp\htdocs\demo\test_error.php on line 9

$sql = "
SELECT * FROM tbl_doc
";
$result = $mysqli->query($sql);
if($result){
	$row = $result->fetch_assoc();
	echo $row['user_v5'];
}

if($result){
	var_dump($result);
	$row = $result->fetch_assoc();
	echo $row['user_v5'];
}

$sql = "
SELECT * FROM tbl_doc2
";
$result = $mysqli->query($sql);
if($result){
	$row = $result->fetch_assoc();
}else{
	die("Error : ".$mysqli->error."<br/>SQL :".$sql);
}

$sql = "
SELECT * FROM tbl_doc2
";
$result = $mysqli->query($sql) or die("Error : ".$mysqli->error."<br/>SQL :".$sql);
if($result){
	$row = $result->fetch_assoc();
}

Error : Table 'test.tbl_doc2' doesn't exist
SQL : SELECT * FROM tbl_doc2

$sql = "
SELECT province_i FROM tbl_provinces
";

Error : Unknown column 'province_i' in 'field list'
SQL : SELECT province_i FROM tbl_provinces

$sql = "
SELECT province_iก FROM tbl_provinces WHERE 
";

Error : You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '' at line 1
SQL : SELECT province_iก FROM tbl_provinces WHERE

$sql = "
SELECT id_a FROM tbl_a,tbl_b WHERE id_a>0 
";

Error : Column 'id_a' in field list is ambiguous
SQL : SELECT id_a FROM tbl_a,tbl_b WHERE id_a>0

$sql = "
SELECT tbl_a.id_a FROM tbl_a,tbl_b WHERE tbl_a.id_a>0 
";

$sql = "
SELECT a.id_a FROM 
tbl_a a,
tbl_b b 
WHERE a.id_a>0 
";

$sql = "
SELECT province_i FROM tbl_provinces  
";
$result = $mysqli->query($sql2) or die("Error : ".$mysqli->error."<br/>SQL :".$sql);
if($result){
	$row = $result->fetch_assoc();
}

Error : 
SQL : SELECT province_i FROM tbl_provinces

Error : Unknown column 'province_i' in 'field list'
SQL : SELECT province_i FROM tbl_provinces