code: public static function getMutation(array &$tokens, $index)
      {
          $tokenCount = count($tokens);
          for ($i=$index+1; $i < $tokenCount; $i++) {
              if (is_array($tokens[$i]) && $tokens[$i][0] == T_WHITESPACE) {
                  continue;
              } elseif (is_array($tokens[$i]) && $tokens[$i][0] == T_LNUMBER) {
                  // Dump the negation - string for negative ints
                  if (self::getPreviousToken($tokens, $i) == '-') {
                      $tokens[$i-1] = [
                          T_WHITESPACE,
                          ''
                      ];
                      break;
                  }
                  // otherwise multiply by -1 to make negative for positive ints
                  $num = (integer) $tokens[$i][1];
                  $replace = -1 * $num;
                  $tokens[$i] = [
                      T_LNUMBER,
                      (string) $replace
                  ];
                  break;
              }
          }
      }
docs: Replace any integer with its sign inverted (i.e. multiply by -1)
  
  @param array $tokens
  @param int $index
  @return array