PHP：在另一个函数中调用函数并使用它的变量[复制]

I want to have something like this stored in a php file:

class SQL
{
 public db_host = "localhost";
 public db_username = "user"; 
 public db_password = "pass";
 public db_databaseName = "myDatabase";

 public function sql_connect()
 {
  $connectDb = new mysqli("$this->db_host", "$this->db_username", "$this->db_password", "$this->db_databaseName");

  if($connectDb->connect_error) {
    exit("Error connecting to database");
  } else {
    echo "Successfully connected to the database";
  }
 }

public function sql_command()
{
 // Call the sql connect method:
 $this->sql_connect();
 << my code to send sql and contains the $connectDb var from the sql_connect() method >>
}
}

I wanted to keep the code used to connect to the database in a separate method so that it can be re-used with multiple methods that contain different sql commands and will all need to connect to the database in order to run.

However, when I run the code the sql_connect method runs but the $connectDb var is not available to the sql_command method and I need it to be.

I have tried various different ways of getting this to work, to no avail.

Any help that can be offered would be appreciated :)

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