Keep connection open in php webservice

Question

I am building a simple webservice and PHP + JSON where I have 2 tables and I intend to perform CRUD operations. It is possible that the webservice is in the medium term consumed by many devices at the same time, which in my opinion can lead to many simultaneous connections. The server has a limit of active connections determined by the people of the infra (I think they are 30). My question is, I use the PDO to open the connection and the connection is called by a static method, in each request I receive the ID of the user who makes the query, Is it better to change the way the connection is performed by always opening and closing each query or will this not affect the number of open connections? Follow an excerpt from my connection class:

class Database extends PDO {

    /**
     * @var array Array of saved databases for reusing
     */
    protected static $instances = array();

    /**
     * Static method get 
     * 
     * @param  array $group
     * @return \helpers\database
     */
    public static function get ($group = false) {
        // Determining if exists or it's not empty, then use default group defined in config
        $group = !$group ? array (
            'type' => 'mysql',
            'host' => 'localhost',
            'name' => 'banco',
            'user' => 'root',
            'pass' => 'toor'
        ) : $group;

        // Group information
        $type = $group['type'];
        $host = $group['host'];
        $name = $group['name'];
        $user = $group['user'];
        $pass = $group['pass'];

        // ID for database based on the group information
        $id = "$type.$host.$name.$user.$pass";

        // Checking if the same 
        if(isset(self::$instances[$id])) {
            return self::$instances[$id];
        }

        try {
            // I've run into problem where
            // SET NAMES "UTF8" not working on some hostings.
            // Specifiying charset in DSN fixes the charset problem perfectly!
            $instance = new Database("$type:host=$host;dbname=$name", $user, $pass);
            $instance->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

            // Setting Database into $instances to avoid duplication
            self::$instances[$id] = $instance;

            return $instance;
        } catch(PDOException $e){
            //in the event of an error record the error to errorlog.html
            Logger::newMessage($e);
            logger::customErrorMsg();
        }
    }

Answer 1

Pertinent question. Part of the answer is in this my answer in the following link here in the OS...

Multiple connections with the Bank

I would add that its implementation contemplates and very well Static array with the connections established in a request. Good solution when a request uses the Database class more than once.

In the case of successive requests there are techniques with reference in server cache but it is not always available. I use the memcached. However I think that optimizing the server to deal with the problem will be the best. As I said gives a reading in my reply I put here the link.