functions example

<?php
$con = NULL;

function connect($host, $user, $password, $db) {
    global $con;
    $con = mysqli_connect($host, $user, $password, $db);
}

function user_exists($username) {
    global $con;
    $query = mysqli_query($con, "SELECT * FROM users WHERE username='".$username."'");
    $userCount = mysqli_num_rows($query);
    if($userCount!=0) {
        return true;
    } else {
        return false;
    }
}

connect("localhost", "root", "", "vaflichka2");

$check = user_exists("tata");
if($check==true) {
    echo "User found!";
} else {
    echo "User not found!";
}
?>