<?php
/**
 * Inverses a provided hex color. If you pass a hex string with a
 * hash(#), the function will return a string with a hash prepended
 *
 * @param string $color Hex color to flip. 
 *    A 3 or 6 character hex code, with or without a leading '#'. 
 *
 * @return string Reversed hex color
 * @author Koncept
 *
 * Last Update: 2020-03-04
 */
function inverse_hex(string $color):string {
  $color = trim($color);
  $prependHash = FALSE;
 
  if (strpos($color, '#') !== FALSE) {
    $prependHash = TRUE;
    $color = str_replace('#', NULL, $color);
  }
 
  switch ($len = strlen($color)) {
    case 3:
      /** @var string $color */
      $color = preg_replace('/(.)(.)(.)/', "\\1\\1\\2\\2\\3\\3", $color);
      break;
    case 6:
      // Already acceptable, passthrough
      break;
    default:
      trigger_error("Invalid hex length ($len). Must be a minimum length of (3) or maxium of (6) characters",
        E_USER_ERROR);
  }
 
  if (!preg_match('/^[a-f0-9]{6}$/i', $color)) {
    throw new RuntimeException(sprintf('Invalid hex string #%s', htmlspecialchars($color, ENT_QUOTES)));
  }
 
  $r = dechex(255 - hexdec(substr($color, 0, 2)));
  $r = (strlen($r) > 1) ? $r : '0' . $r;
  $g = dechex(255 - hexdec(substr($color, 2, 2)));
  $g = (strlen($g) > 1) ? $g : '0' . $g;
  $b = dechex(255 - hexdec(substr($color, 4, 2)));
  $b = (strlen($b) > 1) ? $b : '0' . $b;
 
  return ($prependHash ? '#' : NULL) . $r . $g . $b;
}
 
// Demo
echo inverse_hex('#000'); // #ffffff