Why can't I run query twice in PHP and MySQL?

Question

Below is my code for running a MySQL query from PHP:

mysqli_connect.php

    <?php
        DEFINE ('DB_USER', [my_username]);
        DEFINE ('DB_PASSWORD', [my_password]);
        DEFINE ('DB_HOST', 'localhost');
    
        $databases = array();
        $db_school = @mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, 'db_school')
        OR die('Could not connect to MySQL.db_school');
        $databases['db_school'] = $db_school;
    
        $db_family_finance = @mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, 'db_family_finance')
        OR die('Could not connect to MySQL.db_family_finance');
        $databases['db_family_finance'] = $db_family_finance;
    ?>

get_mysql_data.php

    <?php    
        function query_database($query) {
            require_once('../mysqli_connect.php');
    
            $response = mysqli_query($db_school, $query);
    
            if ($response) {
                echo "ran query!<br>";
            } else {
                echo "couldn't run query!<br>";
            }
        
            return $response;
        }
    
        query_database("SELECT * FROM tests"); //prints out "ran query!"
        query_database("SELECT * FROM tests"); //prints out "couldn't run query!"
    ?>

I am running the same function twice. The first time it returns what I want it to. The second time it doesn't. Why would it do this?

I made a few changes to the code as seen above and I now get this printed to the screen:

ran query!

Notice: Undefined variable: db_school in C:\xampp\htdocs\websites\tutorials\get_mysql_data.php on line 6

Warning: mysqli_query() expects parameter 1 to be mysqli, null given in C:\xampp\htdocs\websites\tutorials\get_mysql_data.php on line 6
couldn't run query!

Answer 1

When you include a file inside a function, the scope of all variables, constants etc are local to the function.

Therefore, the first invocation the mysqli_connect.php is included and everything works.

In the second invocation, the mysqli_connect.php is NOT included because of the require_once . Inside the function, this time, those constants and variables do not exist.

This is an example of how to fix it, depending on what you are doing elsewhere.

Here you declare $dbschool global - this is not generally a good idea.

<?php    
    require_once('../mysqli_connect.php');

    function query_database($query) {
        global $db_school;
        $response = mysqli_query($db_school, $query);

        if ($response) {
            echo "ran query!<br>";
        } else {
            echo "couldn't run query!<br>";
        }

        return $response;
    }

    query_database("SELECT * FROM tests"); //prints out "ran query!"
    query_database("SELECT * FROM tests"); //prints out "couldn't run query!"
?>

or

Here you pass the database to the function

<?php    
    require_once('../mysqli_connect.php');

    function query_database($database, $query) {
        $response = mysqli_query($database, $query);

        if ($response) {
            echo "ran query!<br>";
        } else {
            echo "couldn't run query!<br>";
        }

        return $response;
    }

    query_database($db_school, "SELECT * FROM tests"); //prints out "ran query!"
    query_database($db_school, "SELECT * FROM tests"); //prints out "couldn't run query!"
?>
```php