How to show only json code not with php warning - php

Question

in this case I accidentally made a wrong database username or password, my goal here is how to generate an error in the form of JSON data and open in the form of a PHP warning or error like my picture
// Here's my PHP Code
if (!empty($_POST)){
$test_conn = new mysqli('localhost' , $_POST['db_user'] , $_POST['db_pass']);
$conn = new mysqli('localhost' , $_POST['db_user'] , $_POST['db_pass'] , $_POST['db_name']);
if ($test_conn->connect_error){
$response = array('status' => 0 , 'message' => "Failed Connect to Databases");
print_r(json_encode($response));
}else{
// The Code When Username and Password is Correct
}
}
here I know that the username or password is wrong, but I am confused how to only display the json data in the image at the bottom and not display the php warning

Answer 1

If you are sure that this is what you want, just disable errors by adding the following at the top of your script:
error_reporting(0);
if you want to hide errors for a given code, use the following:
// Disable errors and get current error reporting level
$err = error_reporting(0);
/* here goes you buggy code */
// Set the default error reporting level
error_reporting($err);

Answer 2

You have to suppress the php error reporting in order to use your own check of connect_error. The php documentation on mysqli::$connect_error suggests to do this with the # error control operator like this:
$test_conn = #new mysqli( ... )
This will only suppress errors caused by a faulty instantiation of the mysqli object. It should be clear that you then have to handle these errors yourself (as you already do). All other errors still could cause your php script to stop and not to return any JSON string to the AJAX function that called the script.
As #Victor correctly pointed out in his answer you could disable all errors for the script (or parts of it) by setting error_reporting(0) at the beginning at the script. In that case you wouldn't have to check other possible errors, e.g. if the db credentials in the $_POST array are not set or empty.
As #Ben mentioned in his comments you could also make mysqli throw exceptions to avoid fatal errors by setting mysqli_report(MYSQLI_REPORT_STRICT). This would also be a feasible solution for your problem.
As a general consideration you should make these settings in your php.ini:
display_errors = Off
error_log = syslog
This prevents error message from being exposed to the www (security) while making them available for developers in /var/log/syslog (maintainability).