max_upload_size.php

// Returns the minimum download limit specified in the current php.ini file.
//
// By Andrew Kingdom
// MIT license

function max_upload_size() {
	$max_size = PHP_INT_MAX;
	$post_overhead = 2048; // Reserve 2k for non-file data in the POST.
	
	$tmp = shorthand_bytes(ini_get('upload_max_filesize'));
	if ($tmp > 0 && $tmp < $max_size) $max_size = $tmp;
	
	$tmp = shorthand_bytes(ini_get('post_max_size'));
	if ($tmp > 0 && $tmp < $max_size) $max_size = $tmp;
	
	$tmp = shorthand_bytes(ini_get('memory_limit'));
	if ($tmp > 0 && $tmp < $max_size) $max_size = $tmp;
	
	if ($max_size === PHP_INT_MAX) $max_size = 0;  // no upload limit, use 0, for safety -- comment line out if not required.
	
	return $max_size;
}

function shorthand_bytes($str) {
    $str = trim($str);
    $len = strlen($str);
    if ($len == 0) return 0;
    $last = strtolower($str[$len-1]);
    return round(floatval($str) * pow(1024, stripos('bkmgtpezy', $last)));
}