PHP – Obtain the maximum allowed file upload size

This was actually remarkably difficult to do, in terms of the volume of code required. It uses the fact that there are 3 things which affect the size of a file which can be uploaded: upload_max_filesize, post_max_size and memory_limit. The below code is useful for providing the user with the upload limit on a file upload form.

The code also uses the PHP quirk feature whereby casting a string, which starts with digits, to int will return the int value of the first digits – e.g. (int)’123abc321′ evaluates to 123.

This may, however, be a ridiculous way to do it  and an easier way may exist. Feel free to comment if you know one.

Code block

function max_upload_size() {
    // Multipliers to make byte values from php.ini shorthand
    $suffix_multipliers = array(
        'K'=>1024,
        'M'=>1024*1024,
        'G'=>1024*1024*1024
    );

    // Values to consider
    $vals = array(
        'upload' => ini_get('upload_max_filesize'),
        'post' => ini_get('post_max_size'),
        'mem' => ini_get('memory_limit')
    );

    // Convert above values to bytes based on their shorthand notation
    $byte_vals = array();

    foreach ($vals as $name=>$val) {
        if ($suffix = preg_match('/[a-z]+$/i', $val, $matches)) {
            // "10M" cast to int is 10
            if (isset($suffix_multipliers[$matches[0]])) {
                $byte_vals[$name] = (int)$val * $suffix_multipliers[$matches[0]];
            }
            else {
                $byte_vals[$name] = (int)$val;
            }
        }
    }

    // Get the key(s) of the smallest entries
    $smallest_keys = array_keys($byte_vals, min($byte_vals));
    // Get one of these
    $smallest_key = reset($smallest_keys);

    // Return the smallest
    return $vals[$smallest_key];
}

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