Object to String in PHP

One of my class' function outputs a text, something from my mysql db. But i couldn't explode() it since explode needs a string parameter. How to change it to a string.

class admin{    

//constructor code

public function department_retrieve(){

      //some code to retreive $this->old_department 

    echo $this->old_department;

}
}

$obj = new admin();
$obj->department_retrieve();

$obj->department_retrieve() outputs a text. I want something to explode the text by spaces. What i miss? How to make it a string? Please help.