在PHP中，構建一個平衡的二叉樹可以使用AVL樹。AVL樹是一種自平衡二叉搜索樹，它在每次插入或刪除節點后都會自動調整以保持平衡狀態。以下是一個簡單的PHP實現：

class TreeNode {
    public $value;
    public $left;
    public $right;
    public $height;

    public function __construct($value) {
        $this->value = $value;
        $this->left = null;
        $this->right = null;
        $this->height = 1;
    }
}

class AVLTree {
    private $root;

    private function getHeight($node) {
        if ($node === null) {
            return 0;
        }
        return $node->height;
    }

    private function getBalanceFactor($node) {
        if ($node === null) {
            return 0;
        }
        return $this->getHeight($node->left) - $this->getHeight($node->right);
    }

    private function rightRotate($y) {
        $x = $y->left;
        $T2 = $x->right;

        $x->right = $y;
        $y->left = $T2;

        $y->height = max($this->getHeight($y->left), $this->getHeight($y->right)) + 1;
        $x->height = max($this->getHeight($x->left), $this->getHeight($x->right)) + 1;

        return $x;
    }

    private function leftRotate($x) {
        $y = $x->right;
        $T2 = $y->left;

        $y->left = $x;
        $x->right = $T2;

        $x->height = max($this->getHeight($x->left), $this->getHeight($x->right)) + 1;
        $y->height = max($this->getHeight($y->left), $this->getHeight($y->right)) + 1;

        return $y;
    }

    public function insert($value) {
        $this->root = $this->insertNode($this->root, $value);
    }

    private function insertNode($node, $value) {
        if ($node === null) {
            return new TreeNode($value);
        }

        if ($value < $node->value) {
            $node->left = $this->insertNode($node->left, $value);
        } else if ($value > $node->value) {
            $node->right = $this->insertNode($node->right, $value);
        } else {
            return $node; // Duplicate values are not allowed
        }

        $node->height = 1 + max($this->getHeight($node->left), $this->getHeight($node->right));

        $balanceFactor = $this->getBalanceFactor($node);

        // Left Left Case
        if ($balanceFactor > 1 && $value < $node->left->value) {
            return $this->rightRotate($node);
        }

        // Right Right Case
        if ($balanceFactor < -1 && $value > $node->right->value) {
            return $this->leftRotate($node);
        }

        // Left Right Case
        if ($balanceFactor > 1 && $value > $node->left->value) {
            $node->left = $this->leftRotate($node->left);
            return $this->rightRotate($node);
        }

        // Right Left Case
        if ($balanceFactor < -1 && $value < $node->right->value) {
            $node->right = $this->rightRotate($node->right);
            return $this->leftRotate($node);
        }

        return $node;
    }
}

// Example usage:
$avlTree = new AVLTree();
$avlTree->insert(10);
$avlTree->insert(20);
$avlTree->insert(30);
$avlTree->insert(40);
$avlTree->insert(50);
$avlTree->insert(25);

這個實現包括了一個TreeNode類，用于表示二叉樹的節點，以及一個AVLTree類，用于管理二叉樹的操作。AVLTree類包含了插入節點、左旋轉、右旋轉等方法，以確保樹在每次插入或刪除節點后都能保持平衡。