$ This-> pdo-> exec ($ SQL );

{Code...} shows a source code written in this way. (I omitted a part) Why is it $ this-& amp; gt; pdo-& amp; gt; exec ($ SQL); instead of $ this-& amp; gt; exec ($ SQL); what is the role of this PDO?

class test{      protected $pdo;    function delete($sql){        $this->exec($sql);    }    function exec($sql){        echo $this->pdo->exec($sql);    }}$a=new test;$a->delete('ver');

The source code is written in this way. (I omitted some)

Why is it $ this-> pdo-> exec ($ SQL); instead of $ this-> exec ($ SQL );
What is the role of this PDO?

Reply content:

class test{      protected $pdo;    function delete($sql){        $this->exec($sql);    }    function exec($sql){        echo $this->pdo->exec($sql);    }}$a=new test;$a->delete('ver');

The source code is written in this way. (I omitted some)

Why is it $ this-> pdo-> exec ($ SQL); instead of $ this-> exec ($ SQL );
What is the role of this PDO?

Protected $ pdo; indicates the connection to Data. This variable is required when you need to execute SQL commands.

Estimated _ construct contains $ this-> pdo = new PDO (); and so on.
$ This-> pdo actually points to the PDO class.
$ This is only the current class
The error message above indicates that your $ SQL parameter is not passed.

Because you have not initialized pdo

public function __construct($pdo){    $this->pdo = $pdo;}