Binary Search Tree Implementation

BST.php

<?php
/**
 * Created by PhpStorm.
 * User: kayladnls
 * Date: 12/17/14
 * Time: 5:06 PM
 */

class BinarySearchTree
{
    public $left = NULL;
    public $right = NULL;
    public $value = NULL;

    private function __construct($value)
    {
        $this->value = $value;
    }

    public static function create($value = null)
    {
        return new static($value);
    }

    public function insert($value)
    {
        if ($value == $this->value) return;
        if ($this->value == NULL)
        {
            $this->value = $value;
        }
        else if ($value < $this->value)
        {
            if ($this->left == NULL)
            {
                $this->left = BinarySearchTree::create($value, $this);
            }
            else
            {
                $this->left->insert($value);
            }
        }
        else
        {
            if ($this->right == null)
            {
                $this->right = BinarySearchTree::create($value, $this);
            }
            else
            {
                $this->right->insert($value);
            }
        }
    }

    public function applyDelete()
    {
        $children = $this->countChildren();
        switch ($children)
        {
            case 0:
                return null;
                break;
            case 1:
                return ($this->left != NULL) ? $this->left : $this->right;
                break;
            case 2:
                $this->value = $this->right->pop();
                return $this;
                break;
        }
    }

    public function getMinValueDelete()
    {
        if ($this->value == NULL) return;
        if ($this->left !== NULL && $this->left->hasChildren())
        {
            return $this->left->getMin();
        }
        else if ($this->left !== NULL)
        {
            $min = $this->left->value;
            $this->left = NULL;
            return $min;
        }
        else if ($this->right !== NULL && $this->right->hasChildren())
        {
            return $this->right->getMin();
        }
        else
        {
            $min = $this->right->value;
            $this->right = NULL;
            return $min;
        }
    }

    public function delete($value)
    {
        if ($value == $this->left->value)
        {
            $this->left = $this->left->applyDelete();
        }
        else if ($value == $this->right->value)
        {
            $this->right = $this->right->applyDelete();
        }
        else if ($value < $this->value)
        {
            $this->left->delete($value);
        }
        else
        {
            $this->right->delete($value);
        }
    }

    public function hasChildren()
    {
        return $this->left !== NULL || $this->right !== NULL;
    }

    public function search($value)
    {
        if ($this->value == $value) return $this->value;

        if ($value < $this->value && $this->left != NULL)
        {
            return $this->left->search($value);
        }
        else if ($this->right != NULL)
        {
            return $this->right->search($value);
        }
    }

    public function countChildren()
    {
        $count = 0;
        if ($this->left !== null) $count++;
        if ($this->right !== null) $count++;
        return $count;
    }

    function __toString()
    {
        $output = "";
        $output .= $this->value;
        $output .= "\n";
        $output .= $this->left;
        $output .= $this->right;
        return $output;
    }
}