怎么理解此方法中的单双引号

如何理解此方法中的单双引号

PHP code

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--><?php defined('ACC')||exit('Access denied');

class userModel {
    protected $table = 'user';
    protected $db = false;

    public function __construct() {
        $this->db = mysql::getIns();
    }

    // 用户注册的方法
    public function reg($arr) {
        $sql = "insert into ". $this->table . "(username,passwd,email) values ('" . $arr['username'] . "','" . $arr['passwd'] . "','" . $arr['email'] . "')";
      
       return $this->db->query($sql);
       //print_r(mysql_error());
    }
}

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