Question

How can the file upload size allowed by php settings be determined withing a php script?

Answer 1

The upload is limited by three options: upload_max_filesize, post_max_size and memory_limit. Your upload is only done if it doesn’t exeed one of them.

The ini_get() function provides you with a short hand of the limit and should be converted first. Thx to AoEmaster for this.

function return_bytes($val) {
    $val = trim($val);
    $last = strtolower($val[strlen($val)-1]);
    switch($last) 
    {
        case 'g':
        $val *= 1024;
        case 'm':
        $val *= 1024;
        case 'k':
        $val *= 1024;
    }
    return $val;
}

function max_file_upload_in_bytes() {
    //select maximum upload size
    $max_upload = return_bytes(ini_get('upload_max_filesize'));
    //select post limit
    $max_post = return_bytes(ini_get('post_max_size'));
    //select memory limit
    $memory_limit = return_bytes(ini_get('memory_limit'));
    // return the smallest of them, this defines the real limit
    return min($max_upload, $max_post, $memory_limit);
}

Source: http://www.kavoir.com/2010/02/php-get-the-file-uploading-limit-max-file-size-allowed-to-upload.html