php - How to use PDO with dependency injection? -

i'm having hard time understanding how use dependency injection. i've read on lot of questions/answers here can't picture code i'm using.

model.php

abstract class model {      protected static function getdb() {         static $db = null;          if ($db === null) {             $db = new pdo('mysql:host=host;dbname=dbname;charset=utf8', 'dbuser', 'password');         }          return $db;     } } 

the model.php contains function, want move away setting , calling statically.

user.php

class user extends model {      /*     * selects of user information     */     public function getuser($id){          $db = static::getdb();          $sth = $db->prepare('select * user id = :id');         $sth->bindvalue(':id', $id, pdo::param_int);         $sth->execute();          return $sth->fetch();     }      /*     * selects of user posts     */     public function getuserposts($id){         $db = static::getdb();          $sth = $db->prepare('select * user_posts user_id = :id');         $sth->bindvalue(':id', $id, pdo::param_int);         $sth->execute();          return $sth->fetch();     } } 

in user.php i'm extending model class, have set $db = static::getdb(); in every single function.

i know dependency injection pretty passing methods/variables object i'm not sure i'm doing right.

updated further thoughts:

i'm thinking better create private variable, , in constructor we'd call getdb() so:

class user extends model {      protected $db;      public function __construct(){         $this->db = getdb();     }      /*     * example usage     */     public function getuser($id){         $sth = $this->db->prepare('select * user id = :id');         $sth->bindvalue(':id', $id, pdo::param_int);         $sth->execute();          return $sth->fetch();     } } 

but still count dependency injection i'm not calling class directly in function constructor?

second update: after reading multiple guides, , page wound making lot more sense, came with.

model.php

abstract class model {     protected $db = null;      public function __construct(){         if($this->db === null){             try {                 $this->db = new pdo('mysql:host=' . config::db_host . ';dbname=' . config::db_name . '; charset=utf8', config::db_user, config::db_password);                 $this->db->setattribute(pdo::attr_errmode, pdo::errmode_exception);             } catch (pdoexception $e) {                 echo 'connection failed: ' . $e->getmessage();             }         }         return $this->db;     } } 

user.php

class user extends model {      protected $db;      public function __construct(model $db){         $this->db = $db;     }      /*     * example usage     */     public function getuser($id){         $sth = $this->db->prepare('select * user id = :id');         $sth->bindvalue(':id', $id, pdo::param_int);         $sth->execute();          return $sth->fetch();     } } 

how look?

i think not using dependency injection because not supplying dependencies model, generating them on constructor.

in order supply dependency should pass argument constructor:

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

this way decouple creation of connection class , can use benefits of dependency injection, passing mock object testing instead of actual thing.