Quick Answer

Install older/newer PHP versions on Debian/Ubuntu.

Understanding the Issue

Default repositories may not have all PHP versions. Use third-party repositories like ondrej/php.

The Problem

This code demonstrates the issue:

Bash Error 
# Need PHP 7.4 on Debian 11

The Solution

Here's the corrected code:

Bash Fixed 
# Add ondrej/php PPA
sudo apt install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt update

# Install specific version
sudo apt install php7.4 php7.4-fpm php7.4-mysql

# Switch between versions
sudo update-alternatives --config php

# Verify
php -v

# Install additional extensions
sudo apt install php7.4-{gd,mbstring,xml,zip,curl}

Key Takeaways

Use third-party repositories for non-standard PHP versions.