Static Methods Inside a Class in OOP in PHP

If you want to use static methods inside a class, you should access them not via $this->, but with the help of self::.

As an example, let's add to our Math class a method getDoubleSum, which will find the doubled sum of numbers. We will use the already existing method getSum inside the new method:

<?php
	class Math
	{
		// Find the doubled sum:
		public static function getDoubleSum($a, $b)
		{
			return 2 * self::getSum($a, $b); // use another method
		}
		
		public static function getSum($a, $b)
		{
			return $a + $b;
		}
		
		public static function getProduct($a, $b)
		{
			return $a * $b;
		}
	}
?>

Let's use the new method:

<?php
	echo Math::getDoubleSum(1, 2);
?>

Convert the methods of the following class to static:

<?php
	class ArraySumHelper
	{
		public function getSum1($arr)
		{
			return $this->getSum($arr, 1);
		}
		
		public function getSum2($arr)
		{
			return $this->getSum($arr, 2);
		}
		
		public function getSum3($arr)
		{
			return $this->getSum($arr, 3);
		}
		
		public function getSum4($arr)
		{
			return $this->getSum($arr, 4);
		}
		
		private function getSum($arr, $power) {
			$sum = 0;
			
			foreach ($arr as $elem) {
				$sum += pow($elem, $power);
			}
			
			return $sum;
		}
	}
?>