PHP readfile() returning corrupted data

Question

In my function I am saving an image decoded from a base64 string:

function saveImage(){
    //magic...
    define('UPLOAD_DIR', '../webroot/img/');
    $base64string = str_replace('data:image/png;base64,', '', $base64string);
    $base64string = str_replace(' ', '+', $base64string);
    $data = base64_decode($base64string);
    $id = uniqid();
    $file = UPLOAD_DIR.$id.'.png';
    $success = file_put_contents($file, $data);
}

The above function works properly and the images are saved and not corrupted in the specified folder.

In the next function I am now trying to force download the image to a user:

function getChart($uniqid= null){
    if($uniqid){
        $this->layout = null;
        header("Content-type: image/png");
        header("Content-Disposition:attachment;filename='".$uniqid.".png'");
        readfile('../webroot/img/'.$uniqid.'.png');
        exit;
    } else exit;
}

Image downloaded from the server is corrupted and cant be displayed. After opening the downloaded file in a text editor I noticed that a new line character is added at the very top. After deleting the character and saving the file it opens properly and is being displayed properly.

How can I fix this?

Answer 1

What you describe can have multiple issues that are hidden until you actually open the downloaded file.

Instead make your code more robust and check pre-conditions, here if headers have been send already and to clean any possible existing output buffer and give error if that is not possible:

function getChart ($uniqid = null) {

    if (!$uniqid) exit;

    $this->layout = null;

    if (headers_sent()) throw new Exception('Headers sent.');
    while (ob_get_level() && ob_end_clean());
    if (ob_get_level()) throw new Exception('Buffering is still active.');

    header("Content-type: image/png");
    header("Content-Disposition:attachment;filename='".$uniqid.".png'");
    readfile('../webroot/img/'.$uniqid.'.png');

    exit;
}