PDO Output Parameter Problem

Hi,
I am using this SP, am getting both the result when using mysql workbench.

Code:

---

CREATE PROCEDURE SP(IN _start INT,IN _end INT,INOUT _count INT)
BEGIN

  SET _count = (SELECT COUNT(*) FROM tbl);

  SET @qry = CONCAT('select * from tbl limit ', _start, ',', _end);

  PREPARE stmt FROM @qry;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;

END

---

But when using with PDO am returning this error

PHP Code:

---

$c=0;
$stmt = $this->_dbc->getConnection()->prepare("CALL SP(0,10,:count)");
    $stmt->bindParam(":count",$c,PDO::PARAM_INT|PDO::PARAM_INPUT_OUTPUT,0);
    $stmt->execute();
    return $c; 


---

PDOException' with message 'SQLSTATE[42000]: Syntax error or access violation: 1414 OUT or INOUT argument 3 for routine db22.SP is not a variable or NEW pseudo-variable in BEFORE trigger

But on changing

PHP Code:

---

$this->_dbc->getConnection()->prepare("CALL SP(0,10,**:count**)"); 


---

to

PHP Code:

---

$this->_dbc->getConnection()->prepare("CALL SP(0,10,@count)"); 


---

am not returning any error, but always getting the count as 0.

1.Whats the difference between :count and @count ?
2.How to get exact count via pdo ?