Select from DB with a condicional WHERE

Function Select From DB

<?php
/**
 * Created by Schenier Aldair Lopez
 * Date: 06/12/18
 * Time: 0:53
 *
 */


    function select($tabla,$datos=null, $item=null){

        if ($datos!=null){
            $data = implode(",",$datos);
        }

        if ($datos === null){
            $query = "SELECT * FROM $tabla";
        }

        if ($item!=null){
            if (count($item)<=1){
                $valor = [];
                foreach ($item as $key=>$value) {
                    $columna[]="\"".$value."\"";
                    $valor[]=$key;
                }
                $valores = implode($valor);
                $columnas= implode($columna);
                $query="SELECT $data FROM $tabla WHERE $valores=$columnas";
            }
        }

        if ($datos!=null && $item ==null){
            $query="SELECT $data FROM $tabla";

        }

        $stmt = Conexion::conectar()->prepare($query);
        $stmt->setFetchMode(PDO::FETCH_ASSOC);
        $stmt->execute();
        var_dump($stmt);
    }

?>

select('table_name',['col_1','col_2'],['user'=>'username'])