Queries in php file are not working - php

Question

I've got several functions in my functions.php file. I'd like to add queries to the functions, but when I do that, it doesn't work. It just doesn't show the results of the query. When when I add the query to the main page and include the function, it DOES work.
I'm including the config file, so that is not the problem. I also thought about the scope, so I added GLOBAL $mysqli; to the function, but it still does not work. Any idea what the problem is here? I rather include the queries inside the functions so I do not have to add them to the main page.
My function:
function friends($friendship) {
GLOBAL $mysqli;
$friendship = mysqli_query($mysqli,"SELECT * FROM friends WHERE friend_one = '$my_username' OR friend_one = '$username'
AND friend_two = '$username' OR friend_two = '$my_username'
AND invite_sent = 1 AND invite_accepted = 1 ");
if (mysqli_num_rows($friendship) == 1){
return true;
} else {
return false;
}
}
This is the part to check whether I am friends with someone.
<?php if (friends($friendship) == true) : ?>
We are friends hooray!
<?php endif; ?>
EDIT #1 - COMBINED
So this is how the function looks like after combining the code of the main page with the function:
function friends($friendship) {
$user_id = $_SESSION['user_id'];
//Zoek de username op van ingelogde user
$stmt = $mysqli->prepare("SELECT username, email FROM members WHERE user_id = ? ");
$stmt->bind_param('i', $user_id);
$stmt->execute();
$stmt->store_result();
$stmt->bind_result($my_username, $my_email);
$stmt->fetch();
$stmt->close();
$username = $_GET["username"];
//Zoek de gebruiker en zijn/haar gegevens in de db
$stmt = $mysqli->prepare("SELECT user_id, gender, email, protected FROM members WHERE username = ? ");
$stmt->bind_param('s', $username);
$stmt->execute();
$stmt->store_result();
$stmt->bind_result($userid, $gender, $email, $protected);
$stmt->fetch();
$stmt->close();
$friendship = mysqli_query($mysqli,"SELECT * FROM friends WHERE friend_one = '$my_username' OR friend_one = '$username'
AND friend_two = '$username' OR friend_two = '$my_username'
AND invite_sent = 1 AND invite_accepted = 1 ");
if (mysqli_num_rows($friendship) == 1){
return true;
} else {
return false;
}
}

Answer 1

How about making sure all of your variables are available in the query?
function friends($friendship, $my_username, $username) {
//Added $my_username, $username to the function call
//since they are on the main page but not declared in the function otherwise
GLOBAL $mysqli;
$friendship = mysqli_query($mysqli, "SELECT * FROM friends WHERE (friend_one = '$my_username' OR friend_one = '$username')
AND (friend_two = '$username' OR friend_two = '$my_username')
AND (invite_sent = 1 AND invite_accepted = 1)") or die(mysqli_error($mysqli)); //Throw an error if the query fails
if (mysqli_num_rows($friendship) == 1){
return true;
} else {
return false;
}
}
I would also caution against using the GLOBAL and I'd add on some error checking to help diagnose problems.
<?php
//Add the variables here as well...
$friends = friends($friendship, $my_username, $username);
if($friends == true)
echo 'We are friends hooray!';
} else {
//Do something?
};
?>

Answer 2

For all of the variables that your function will use, like $my_username in this case, you will have to either declare and initialize them inside the function or pass them in the form of the parameters to this function like #jason has described in the answer.
The reason why these queries are working on your main page is that your code(the query) is able to access variables since they are defined in that particular scope.
Since this function 'friends' has no idea about what $my_username is, you will simply need to either define the variables in this function or pass them in the form of parameters.