如何从php脚本打印到apache日志(access.log)

我正在开发一个带有php后端的iOS应用程序，我有一个php服务，它正在处理文件上传，我从互联网上获得了代码，但它不起作用。我试图通过使用print语句来调试它，但它没有打印任何东西到access.log，而是打印到屏幕上。有人能告诉我，如何打印输出访问。日志从php脚本。这里是php服务器-

 <?php
$firstName = $_POST["firstName"];
$lastName = $_POST["lastName"];
$userId = $_POST["userId"];

echo(var_dump($_POST));
$target_dir = "wp-content/uploads/2015/02";if(!file_exists($target_dir))
{
mkdir($target_dir, 0777, true);
}
$target_dir = $target_dir . "/" . basename($_FILES["file"]["name"]);
if (move_uploaded_file($_FILES["file"]["tmp_name"], $target_dir))
{
echo json_encode([
"Message" => "The file ". basename( $_FILES["file"]["name"]). " has been uploaded.",
"Status" => "OK",
"userId" => $_REQUEST["userId"]
]);
} else {
echo json_encode([
"Message" => "Sorry, there was an error uploading your file.",
"Status" => "Error",
"userId" => $_REQUEST["userId"]
]);
}
?>