WWW POST gives me Internal Server Error

I'm using Unity to make a button that takes a screenshot and then uploads it to my server.
The point is that if I send a form to the server, the WWW gives me an Internal Server Error.
I've tried a lot of examples from the internet and it still gives me that error. Every time.

This is my C# code for uploading the screenshot:

IEnumerator uploadPhoto(){
    /*yield return new WaitForEndOfFrame();
    Texture2D snap = new Texture2D (webCamTexture.width, webCamTexture.height);
    snap.SetPixels (webCamTexture.GetPixels ());
    snap.Apply ();
    webCamTexture.Stop();*/

    yield return new WaitForEndOfFrame();
    Texture2D snap = new Texture2D(Screen.width, Screen.height);
    snap.ReadPixels(new Rect(0,0,Screen.width,Screen.height),0,0);
    snap.Apply();

    byte[] bytes = snap.EncodeToPNG();

    WWWForm form = new WWWForm();
    form.AddField("username", PlayerPrefs.GetString("username"));
    form.AddBinaryData("form_file", bytes, "screenshot.png", "image/png");
    //Debug.Log(System.BitConverter.ToString(bytes));

    WWW connection = new WWW(url, form); 
    yield return connection;

    if (connection.error != null)
    {
        Debug.Log("Server-side error: " + w.error);
    }
    else
    {
        Debug.Log(connection.text);
    }
}

And this is the php script made to receive that BinaryData:

if ($_POST){
    if ($_FILES["form_file"]["error"] !== UPLOAD_ERR_OK) {
        die("Upload failed with error code " . $_FILES["form_file"]["error"]);
    } else {
        if ((($_FILES["form_file"]["type"] == "image/jpg") || ($_FILES["form_file"]["type"] == "image/jpeg") || ($_FILES["form_file"]["type"] == "image/png")) && ($_FILES["form_file"]["size"] < 20000000000)){
            if ($_FILES["form_file"]["error"] > 0) {
                echo "File error: " . $_FILES["form_file"]["error"] . ""; 
            }else{ 

                echo "Uploaded image: " . $_FILES["form_file"]["name"] . "<br>";
                echo "Type: " . $_FILES["form_file"]["type"] . "<br>";
                echo "Size: " . ($_FILES["form_file"]["size"] / 1024) . " Kb<br>";
                echo "Temporary name: " . $_FILES["form_file"]["tmp_name"] . "<br>";

                if (file_exists("upload/" . $_FILES["form_file"]["name"])){
                    echo $_FILES["form_file"]["name"] . " already exists. ";
                }else{
                    move_uploaded_file($_FILES["form_file"]["tmp_name"], "upload/" . $_FILES["form_file"]["name"]);
                    echo "Stored in: " . "upload/" . $_FILES["form_file"]["name"];
                }
            }
        } else{ 
        echo "Invalid file"; 
        }
    }
} else{
    echo "No POST. ";
}

I don't know what's wrong with this. I'm trying to make this work for 2 days now. I just know that the error is on the server-side. I tried to change the permissions of the script to 777, no effect.
What am I doing wrong?