Below is the instalalation plan:

1. Launch an EC2 Instance: Follow the step-by-step instructions to launch an EC2 instance with Amazon Linux 2023 or Amazon Linux 2. Ensure you select an appropriate instance size based on your application's requirements.

2. Update the System: After launching the instance, update the system packages and dependencies to ensure you have the latest software versions and security patches.

3. Install Apache: Install the Apache web server using the package manager. Configure the necessary virtual hosts and directories to host your PHP applications securely.

4. Install PHP: Install PHP and its required modules using the package manager. Customize the PHP configuration to meet your application's needs, such as memory limits, file upload sizes, and error logging.

5. Configure Apache for PHP: Configure Apache to work seamlessly with PHP. Enable the PHP module and define the necessary PHP handlers in the Apache configuration.

6. Install and Secure MariaDB: Install MariaDB, a powerful and reliable open-source database management system. Follow the best practices for securing MariaDB, including setting strong passwords, restricting remote access.

7. Optimize PHP and Apache: Fine-tune the PHP and Apache configurations to enhance performance. Remove unnecessary modules. Adjust parameters such as memory limits, caching mechanisms, and connection limits to optimize resource utilization and response times.

8. Enable SSL/TLS Encryption: Secure your web applications by enabling Letsencrypt Free SSL/TLS encryption. Obtain and install an SSL certificate and schedule cron job to auto renew it. Configure Apache to enforce HTTPS connections to protect sensitive data.

9. Implement Firewall and Network Security: Configure the built-in firewall (iptables) or use AWS security groups to restrict incoming and outgoing network traffic. Employ network-level security practices, such as limiting access to specific ports and enabling brute-force protection.

10. Monitor and Scale (optional): Implement monitoring tools like Amazon CloudWatch to keep track of resource utilization, performance metrics, and security events. Prepare for scalability by utilizing Amazon Elastic Load Balancer to distribute traffic efficiently and horizontally scale your PHP application stack.

This cost-effective and optimized solution allows you to leverage the power of AWS EC2 Instances while ensuring optimal security and performance for your web applications. Enjoy the benefits of a scalable and efficient infrastructure on the Amazon Web Services (AWS) platform!