How do I pass a database connection into a method PHP

I have a class in PHP and one of the methods needs to access a database.How do I correctly pass a database connection variable to a method? Does it get passed as a parameter through the constructor or method? Or is it something completely different?

Check the following updated answer. Tested and working.

  <?php

class SomeClass
{


    function setDb($servername, $username, $password, $database)
    {
        // Create the database connection and use this connection in your methods as $this->conn
        $this->conn = new mysqli($servername, $username, $password, $database);
        if ($this->conn->connect_error) {
            die("Connection failed: " . $this->conn->connect_error);
        }
        echo "New successful connection to myDb  \n";
    }

    public function createTable()
    {
        // sql to create table
        $sql = "CREATE TABLE MyGuests (
    id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    firstname VARCHAR(30) NOT NULL,
    lastname VARCHAR(30) NOT NULL,
    email VARCHAR(50),
    reg_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
    )";

        if ($this->conn->query($sql) === TRUE) {
            echo "New table created successfully \n";
        } else {
            echo "Error: " . $sql . "<br>" . $this->conn->error;
        }
    }
    public function normalInsertDb()
    {
        // sql to insert record using normal query
        $sql = "INSERT INTO MyGuests (firstname, lastname, email)
        VALUES ('John', 'Doe', 'john@example.com')";

        if ($this->conn->query($sql) === TRUE) {
            echo "New record inserted successfully using normal sql statement \n";
        } else {
            echo "Error: " . $sql . "<br>" . $this->conn->error;
        }
    }
    public function preparedInsertDb()
    {
        // prepare and bind
        $stmt = $this->conn->prepare("INSERT INTO MyGuests (firstname, lastname, email) VALUES (?, ?, ?)");
        $stmt->bind_param("sss", $firstname, $lastname, $email);
        // set parameters and execute
        $firstname = "John";
        $lastname = "Doe";
        $email = "john@example.com";
        $stmt->execute();

        $firstname = "Mary";
        $lastname = "Moe";
        $email = "mary@example.com";
        $stmt->execute();

        $firstname = "Julie";
        $lastname = "Dooley";
        $email = "julie@example.com";
        $stmt->execute();

        echo "New records inserted successfully using PREPARED STATEMENTS \n";

        $stmt->close();
        $this->conn->close();
    }
}

$obj = new SomeClass();
$obj->setDb('localhost', 'homestead', 'secret', 'myDb'); //we assume the database myDb exists
$obj->createTable();
$obj->normalInsertDb();
$obj->preparedInsertDb();

My Result: