getimagesize（）：文件不能为空

I copied the following script from w3schools.com small pictures are uploading fine. But Big size pictures are not uploading even I changes file size also but when I upload pcitures it says file is not an image where file is jpg file and it's 3.98 MB file dimesion 3500X4691. It shows

Warning: getimagesize(): Filename cannot be empty

looking forward for help :) I also try to define MAX file size at top of file but it also don't work. I want to upload image of upto 10 MB

<?php


$target_dir = "../uploads/";
$target_file = mysqli_real_escape_string($db, $target_dir . basename($_FILES["fileToUpload"]["name"]));
$uploadOk = 1;
$imageFileType = pathinfo($target_file,PATHINFO_EXTENSION);
// Check if image file is a actual image or fake image
if(isset($_POST["submit"])) {
    $check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);
    if($check !== false) {
        echo "File is an image - " . $check["mime"] . ".";
        $uploadOk = 1;
    } else {
        echo "File is not an image.";
        $uploadOk = 0;
    }
}
// Check if file already exists
if (file_exists($target_file)) {
    echo "Sorry, file already exists.";
    $uploadOk = 0;
}
// Check file size
if ($_FILES["fileToUpload"]["size"] > 10485760) {
    echo "Sorry, your file is too large.";
    $uploadOk = 0;
}
// Allow certain file formats
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
&& $imageFileType != "gif" ) {
    echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
    $uploadOk = 0;
}
// Check if $uploadOk is set to 0 by an error
if ($uploadOk == 0) {
    echo "Sorry, your file was not uploaded.";
// if everything is ok, try to upload file
} else {





        $filename = $target_dir. $now() .$target_file;  


    if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $filename)) {
        echo  "okk";

        $upload_link = $target_dir . $filename;
    } else {
        echo "Sorry, there was an error uploading your file.";
    }
}
 ?>