PHP

pdo close connection

$conn=new PDO("mysql:host=$host;dbname=$dbname",$user,$pass);
    // If this is your connection then you have to assign null
    // to your connection variable as follows:
$conn=null;
    // By this way you can close connection in PDO.

close connection pdo

$pdo = null;

pdo close connection

$this->connection = new PDO();
$this->connection->query('KILL CONNECTION_ID()');
$this->connection = null;