Question

I am quite familiar with PHP but I have just started doing a bit of object oriented stuff with it. I wanted to make a singleton database connection but I ran into a problem and error.”Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 523800 bytes) in”
I know that I am not supposed to run out of memory the query I am running is

$con = getConnection();
$stmt = $con->prepare("SELECT gene_name,jgi_protein_id FROM   jgi_transcriptid_proteinid_match where our_protein_id = ?");

Here is the code for the class.

class Connection
{
    // Store the single instance of connection 
    private static $connection;

    private function __construct()
    {
        $connection = new mysqli(HOSTNAME, DBUSER, PASSWORD, DBNAME);

        if ($connection->connect_errno)
            die("Failed to connect to MySQL: (" . $connection->connect_errno . ") " . $connection->connect_error);
    }

    public static function getInstance() 
    { 
        if (!self::$connection) 
            self::$connection = new Connection(); 

        return self::$connection; 
    }

    public function prepare($query) 
    {
        $statement = $this->prepare($query);
        return $statement; 
    }
}

I am using mysqli for the database stuff.

Answer 1

There are a couple of issues in this code:

Infinite recursion

public function prepare($query) 
{
  $statement = $this->prepare($query);
  return $statement; 
}

Referencing a local variable, instead of the static one
The code should probably reference self::$connection. Based on the class, though, I’m not sure, as self::$connection is used in a different way in getInstance(), which I don’t see called anywhere.

private function __construct()
    {
        $connection = new mysqli(HOSTNAME, DBUSER, PASSWORD, DBNAME);

        if ($connection->connect_errno)
            die("Failed to connect to MySQL: (" . $connection->connect_errno . ") " . $connection->connect_error);
    }

Naming confusion
The class is called Connection, and it contains a static variable named $connection, which stores the singleton instance of the class. The constructor contains another $connection, which is instead a mysqli connection.

Refactored class – UNTESTED FIX
The class below has not been tested and is provided for illustrative purpose. Use at your own risk.

class Connection {
// Store the single instance of the class
private static $instance;
// Store the mysqli connection
private $connection;

public function __construct() {
    // NOTE: it would be better to pass connection parameters as arguments
    $this->connection = new mysqli(HOSTNAME, DBUSER, PASSWORD, DBNAME);

    if ($this->connection->connect_errno)
        die("Failed to connect to MySQL: (" . $this->connection->connect_errno . ") " . $this->connection->connect_error);
}

public static function getInstance() { 
    if(empty(self::$instance)) {
        self::$instance = new Connection(); 
    }

    return self::$instance; 
}

public function prepare($query) {
    $statement = $this->connection->prepare($query);
    return $statement; 
}

}