php - What to do with mysqli problems? Errors like mysqli_fetch_array(): Argument #1 must be of type mysqli_result and such

Solution:

TL;DR

1. Always havemysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT); in your mysqli connection code and always check the PHP errors.
2. Always replace every PHP variable in the SQL query with a question mark, and execute the query using . It will help to avoid syntax errors of all sorts.

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
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Here is a gist of my article on PHP error reporting: Reporting errors on a development and live servers must be different. On the development server it is convenient to have errors shown on-screen, but on a live server error messages must be logged instead, so you could find them in the error log later.

Therefore, you must set corresponding configuration options to the following values:

• On a development server

• error_reporting should be set toE_ALL value;

• log_errors should be set to 1 (it is convenient to have logs on a development PC too)

• display_errors should be set to 1

• On a production server

• error_reporting should be set toE_ALL value;

• log_errors should be set to 1

• display_errors should be set to 0

After that, when MySQL query fails, you will get a PHP error that explains the reason. On a live server, in order to get the error message, you'll have to check the error log.

How to actually use it

Just remove any code that checks for the error manually, all thoseor die(),if ($result),try..catch and such. Simply write your database interaction code right away:

$stmt = $this->con->prepare("INSERT INTO table(name, quantity) VALUES (?,?)");
$stmt->bind_param("si", $name, $quantity);
$stmt->execute();
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Again, without any conditions around. If an error occurs, it will be treated like any other error in your code. For example, on a development PC it will just appear on-screen, while on a live site it will be logged for the programmer, whereas for the user's convenience you could use an error handler (but that's a different story which is off topic for MySQLi, but you may read about it in the article linked above).

What to do with the error message you get

First of all you have to locate the problem query. The error message contains the file name and the line number of the exact spot where the error occurred. For the simple code that's enough, but if your code is using functions or classes you may need to follow the stack trace to locate the problem query.

After getting the error message, you have to read and comprehend it. It sounds too obvious if not condescending, but learners often overlook the fact that the error message is not just an alarm signal, but it actually contains a detailed explanation of the problem. And all you need is to read the error message and fix the issue.

• Say, if it says that a particular table doesn't exist, you have to check spelling, typos, and letter case. Also you have to make sure that your PHP script connects to a correct database
• Or, if it says there is an error in the SQL syntax, then you have to examine your SQL. And the problem spot is right before the query part cited in the error message.

If you don't understand the error message, try to google it. And when browsing the results, stick to answers that explain the error rather than bluntly give the solution. A solution may not work in your particular case, but the explanation will help you to understand the problem and make you able to fix the issue by yourself.

You have to also trust the error message. If it says that number of tokens doesn't match the number of bound variables then it is so. The same goes for the absent tables or columns. Given the choice, whether it's your own mistake or the error message is wrong, always stick to the former. Again it sounds condescending, but hundreds of questions on this very site prove this advise extremely useful.

A list of things you should never ever do in regard of error reporting

• Never use an error suppression operator (@)! It makes a programmer unable read the error message and therefore unable to fix the error
• Do not usedie() orecho or any other function to print the error message on the screen unconditionally. PHP can report errors by itself and do it the right way depends on the environment - so just leave it for PHP.
• Do not add a condition to test the query result manually (likeif($result)). With error exceptions enabled such condition will just be useless.
• Do not use thetry..catch operator for echoing the error message. This operator should be used to perform some error handling, like a transaction rollback. But never use it just to report errors - as we learned above, PHP can already do it, the right way.

P.S.
Sometimes there is no error, but no results either. Then it means, there is no data in the database to match your criteria. In this case you have to admit this fact, even if you can swear the data and the criteria are all right. They are not. You have to check them again.

I've got an article that can help in this matter, How to debug database interactions. Although it is written for PDO, the principle is the same. Just follow those instructions step by step and either have your problem solved or have an answerable question for Stack Overflow.