simplified $placeholder form
<?php
$data = ['a'=>'foo','b'=>'bar'];
$keys = array_keys($data);
$fields = '`'.implode('`, `',$keys).'`';
#here is my way
$placeholder = substr(str_repeat('?,',count($keys)),0,-1);
$pdo->prepare("INSERT INTO `baz`($fields) VALUES($placeholder)")->execute(array_values($data));