Function for database connection with a static variable

Question

Source: phptutorial.net/php-tutorial/php-registration-form/

Function db() connects the database once and returns a new PDO object with the configuration defined in another file.

function db(): PDO
{
    static $pdo;

    if (!$pdo) {
        return new PDO(
            sprintf("mysql:host=%s;dbname=%s;charset=UTF8", DB_HOST, DB_NAME),
            DB_USER,
            DB_PASSWORD,
            [PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION]
        );
    }
    return $pdo;
} 

The site says during the first call $pdo is not defined hence the statement inside if is executed and new PDO object is returned and during the second call the PDO object is returned since $pdo is a static variable. How does it not return a new PDO object again during the second call? Even if $pdo is static we have not assigned the PDO object to it. Wont it retain its null value?