code: public static function MAX(...$args)
      {
          $returnValue = null;
  
          // Loop through arguments
          $aArgs = Functions::flattenArray($args);
          foreach ($aArgs as $arg) {
              // Is it a numeric value?
              if ((is_numeric($arg)) && (!is_string($arg))) {
                  if (($returnValue === null) || ($arg > $returnValue)) {
                      $returnValue = $arg;
                  }
              }
          }
  
          if ($returnValue === null) {
              return 0;
          }
  
          return $returnValue;
      }
docs: MAX.
  
  MAX returns the value of the element of the values passed that has the highest value,
  with negative numbers considered smaller than positive numbers.
  
  Excel Function:
  MAX(value1[,value2[, ...]])
  
  @category Statistical Functions
  
  @param mixed ...$args Data values
  
  @return float