Heap in Class

HeapInClass.php

<?php

  class Heap {

	private $items = [];

	public function peek() {
		if ( $this->items ) {
			return $this->items[0];
		}

		return null;
	}

	public function add( $item ): void {
		$this->items[] = $item;

		$current = count( $this->items ) - 1;
		while( $this->getParent( $current ) && $this->getParent( $current ) < $this->items[ $current ] ) {
			$this->swap( $current, $this->getParentIndex( $current ) );
			$current = $this->getParentIndex( $current );
		}
	}

	public function remove(): void {
		unset( $this->items[0] );

		if ( ! $this->items ) {
			return;
		}

		$lastIndex = count( $this->items );
		array_unshift( $this->items, $this->items[ $lastIndex ] );
		unset( $this->items[ $lastIndex ] );

		$current = 0;
		while ( $this->getLeft( $current ) ) {
			$left  = $this->getLeft( $current );
			$right = $this->getRight( $current );

			if ( $left > $right && $left > $this->items[ $current ] ) {
				$leftIndex = $this->getLeftIndex( $current );
				$this->swap( $current, $leftIndex );
				$current = $leftIndex;
			} elseif ( $right > $left && $right > $this->items[ $current ] ) {
				$rightIndex = $this->getRightIndex( $current );
				$this->swap( $current, $rightIndex );
				$current = $rightIndex;
			}
		}
	}

	private function swap( $indexOne, $indexTwo ): void {
		$temp                     = $this->items[ $indexOne ];
		$this->items[ $indexOne ] = $this->items[ $indexTwo ];
		$this->items[ $indexTwo ] = $temp;
	}

	private function getLeftIndex( $index ) {
		return ( $index * 2 + 1 );
	}

	private function getRightIndex( $index ) {
		return $index * 2 + 2;
	}

	private function getParentIndex( $index ) {
		return (int)( ( $index - 1 ) / 2 );
	}

	private function getLeft( $index ) {
		return isset( $this->items[ $this->getLeftIndex( $index ) ] ) ? $this->items[ $this->getLeftIndex( $index ) ] : null;
	}

	private function getRight( $index ) {
		return isset( $this->items[ $this->getRightIndex( $index ) ] ) ? $this->items[ $this->getRightIndex( $index ) ] : null;
	}

	private function getParent( $index ) {
		return isset( $this->items[ $this->getParentIndex( $index ) ] ) ? $this->items[ $this->getParentIndex( $index ) ] : null;
	}
}

$heap = new Heap();
$heap->add( 10 );
$heap->add( 20 );
$heap->add( 30 );
$heap->add( 40 );
$heap->remove();

echo $heap->peek();