Добавление в строку значения, которого в ней ещё нет

    public function addBasket($cat,$id){
        $this->basket = $this->returnBasket();
        if(!empty($this->basket[$cat])){
             if($this->basket[$cat]!=$id)
            $this->basket[$cat].= ",$id";
        }else{
            $this->basket[$cat] = "$id";
        } 
        $this->saveBasket();
    }

if(array_search($id, explode(',',$this->basket[$cat])) !== false)
    $this->basket[$cat].= ",$id";

             public function addBasket($cat,$id){
        $this->basket = $this->returnBasket();
        if(!empty($this->basket[$cat])){
            if(in_array($id,explode(",",$this->basket[$cat]),true)== false)
            $this->basket[$cat].= ",$id";
        }else{
            $this->basket[$cat] = "$id";
        } 
        $this->saveBasket();
    }

if(!in_array($id,explode(",",$this->basket[$cat])))

var_dump(in_array(0, ['key' => 'value'])); // bool(true)
var_dump(in_array(0, ['key' => 'value'], true)); // bool(false)

var_dump(in_array(12, ['11', '12', '13'], true)); // bool(false)

public function addBasket($cat, $id) {
    $this->basket = $this->returnBasket();
    if (empty($this->basket[$cat]))
        $this->basket[$cat] = [];
    if (!in_array($id, $this->basket[$cat], true))
        array_push($this->basket[$cat], $id);
    $this->saveBasket();
}