PDO insert multiple value

<?php
//---
function placeholders($text, $count=0, $separator=","){
    $result = array();
    if($count > 0){
        for($x=0; $x<$count; $x++){
            $result[] = $text;
        }
    }

    return implode($separator, $result);
}
$pdo->beginTransaction() // also helps speed up your inserts
$insert_values = array();
foreach($data as $d){
 $question_marks[] = '('  . placeholders('?', sizeof($d)) . ')';
 $insert_values = array_merge($insert_values, array_values($d));
}
$sql = "INSERT INTO table (" . implode(",", array_keys($datafield) ) . ") "
      ."VALUES " . implode(',', $question_marks);
$stmt = $pdo->prepare ($sql);
try {
    $stmt->execute($insert_values);
} catch (PDOException $e){
    echo $e->getMessage();
}
$pdo->commit();