How to properly require PHP files

Question

I'm using 2 main PHP files for scripts on my website. One that focuses on accessing data from a MySQL database and one that contains functions required for the website. In some functions (in the functions file) I need to call functions from my MySQL file. I don't understand how to properly require the files so they are usable when they are necessary.

include 'scripts/MySQL.php';
include 'scripts/functions.php';

Is at the top of the page's file (I use functions from both files directly). Then when I need to use MySQL functions in functions.php , the MySQL functions are null if I don't require MySQL.php at the top of the functions.php file, but if I do, I get this error: Fatal error: Cannot redeclare getUserInfo() (previously declared in .../scripts/MySQL.php:2) in .../scripts/MySQL.php on line 16 . This is getUserInfo(line 2 is the first line posted, and line 16 is the last):

function getUserInfo($steamID) {
    $link = mysqli_connect(constant('DB_SERVER'), constant('DB_USER'), constant('DB_PASS'), constant('DB_NAME'));

    /* check connection */
    if (mysqli_connect_errno()) {
        printf("Connect failed: %s\n", mysqli_connect_error());
        exit();
    }

    $query = 'SELECT * FROM '.constant('membersTable').' WHERE steamID = '.$steamID;

    if($result = mysqli_query($link, $query)) {
        return mysqli_fetch_assoc($result);
    }
}

How does require really work with inter-file dependencies? And why does my code act so weird/how do I fix it?

Also, if I want to access variables from another file, do I have to require that file in every function that uses the variables, or can I require it at the top of the file containing the functions (the latter does not seem to work)?

Sorry if this question reads a little confusing. I really could not figure out a good way to word my problem (especially cause I'm not sure I understand the problem myself).

Answer 1

use include_once or require_once to avoid redeclaring your functions

require_once 'scripts/MySQL.php';
require_once 'scripts/functions.php';

The require_once statement is identical to require except PHP will check if the file has already been included, and if so, not include (require) it again.