Checks a number and returns true if it validates according to Luhn Algorithm

luhn.php

/**
* Valid Luhn Number Checker
*/

class Luhn 
{
  
	static public function valid($num)
	{
		$odd  = 0;
		$even = 0;

		// reverse string
		$str = strrev($num);
		
		$strArr = str_split($str);

		foreach ($strArr as $k => $n) {
			if (self::_isOdd($k)) {
				// odd
				$odd += $n;
			}else{
				// even
				$n = $n * 2;
				$even += self::_strSum($n);
			}
		}

		return ($odd + $even) % 10 ? false : true;
	}

	protected static function _isOdd($num)
	{
		return (bool) !($num % 2);
	}

	protected static function _strSum($num)
	{
		if(strlen($num) > 1){
			return (substr($num, 0, 1) + substr($num, 1));
		}else{
			return $num;
		}
	}
}