$this->pdo->exec ($sql); what does that mean?

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

See a source code is written in this way. (I omitted a part)

Why is $this->pdo->exec ($sql); Rather than $this->exec ($sql);
What's the effect of adding 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');

See a source code is written in this way. (I omitted a part)

Why is $this->pdo->exec ($sql); Rather than $this->exec ($sql);
What's the effect of adding this PDO?

protected $pdo; The connection to the data is represented. When you need to execute a SQL command, of course you need to use this variable AH.

__construct is estimated to have $this->pdo = new PDO ();
$this->pdo is actually pointing to the PDO class.
$this is just the current class
The above your error message is that your $sql this parameter is not transmitted

Because you didn't initialize PDO.

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

