Most Popular PHP Extensions

There are many PHP extensions that are widely used in various web development projects, but some of the most popular ones include:

1. PDO (PHP Data Objects) - allows developers to interact with various database management systems such as MySQL, PostgreSQL, and Oracle.

2. cURL - enables communication with other servers using various protocols such as HTTP, HTTPS, FTP, and more.

3. mbstring - provides support for multibyte strings, which allows for the manipulation of text encoded in different character sets.

4. gd - enables the creation and manipulation of images, including JPEG, PNG, and GIF formats.

5. openssl - provides support for secure socket layer (SSL) and transport layer security (TLS) protocols, which are commonly used for secure communication over the internet.

6. json - provides support for JSON (JavaScript Object Notation), which is a lightweight data interchange format that is easy for humans to read and write and easy for machines to parse and generate.

7. xdebug - a powerful debugging and profiling tool for PHP, it allows developers to identify and fix errors in their code more easily

8. intl - enables internationalization support for PHP, which provides functions for working with different languages and character sets.
   
   Keep in mind that the popular extensions used can vary depending on the project, server or framework you are using.
   
   

   To install PHP extensions on a Linux server, you can use the command line. The process typically involves the following steps:

   1. Use the package manager of your Linux distribution (apt, yum, dnf etc) to install the necessary dependencies for the extension you want to install.

   For example:

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   sudo apt-get install -y libssl-dev libcurl4-openssl-dev
   2. Use the package manager to install the extension package.

   For example:

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   sudo apt-get install -y php-curl
   3. Enable the extension by adding the appropriate line to your php.ini file.

   For example:

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   extension=curl.so
   4. Restart the web server for the changes to take effect.

   For example:

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   sudo service apache2 restart

   Note: This is a general guideline, the commands may vary depending on the Linux distribution, version and PHP version you're using.

   Also, it's important to note that, some extensions are already built-in in the PHP package and you don't need to install them separately, you just need to enable them.

   You should check the PHP version you're using and the package manager you have in your Linux distribution to have the correct commands.