如何在PHP中添加时间？

I fetch various rows from the database (the column is formatted as datetime) and formatted it to time only. Now I want to add all the row into one PHP variable, how can I achieve this?

 $total_time = $this->fetchTime();

 public function fetchTime(){
    $catch = 0;
    $db = $this->getConn();
    $statement = $db->prepare("SELECT log_in FROM attendance");
    $statement->execute();
    while ($result = $statement->fetch()){
       $catch += $result[0]; //This is the part where I do not know how to add time
    }
    return $catch;

 }

this might help you

public function fetchTime(){
    $catch = "";
    $db = $this->getConn();
    $statement = $db->prepare("SELECT log_in FROM attendance");
    $statement->execute();
    while ($result = $statement->fetch()){
       $catch.= $result[0]; //This is the part where I do not know how to add time
    }
    return $catch;

 }