There are two options.

1. Access the php file through a local webserver(ie thru a local website). The web-server will deal with the requested php file. It will use either,

   • Inbuilt PHP module to interpret the php file, or

   • PHP through CGI (eg.CGI, FastCGI)
     
     If your apache(check if apache is running using service apache2 status!!) is set to the default configuration, this could be as simple as
     

     http://localhost/path/to/your.php
     

     Remember by default, the base directory for apache is /var/www/html/, so you need not include this in the url.

2. Use the php binary directly from a terminal.

      php /path/to/your/file.php
   

You need to install the php5-cli or php5-cgi package.

sudo apt-get install php5-cli
# OR
sudo apt-get install php5-cgi

As Zoredache noted in the comment. cli version doesn't process headers nor dos output them - it's sort of clean PHP interpreter completely unaware of HTTP.

If you want version capable of above mentioned, use the CGI version.

I really recommend installing PHP via Ondřej Surý's PPA

sudo add-apt-repository ppa:ondrej/php
sudo apt update
sudo apt search php

You will be able not only to install PHP but also to install most of its popular PECL packages.

Please install software-properties-common first

sudo apt-get install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update

After the above ,

then either execute

sudo apt-get install -y php7.4

or, if you wish

sudo apt-get install -y php7.4 php7.4-cli php7.4-json php7.4-common php7.4-mysql php7.4-zip php7.4-gd php7.4-mbstring php7.4-curl php7.4-xml php7.4-bcmath

Additional Comments

if your apt is out-of-date, please execute the following 2 lines first, before installing software-properties-common

$ sudo apt update
$ sudo apt upgrade

If unfortunatley the binaries are removed (it happens) you may need to either (1) compile it (the old software version) by yourself, or (2) upgrade your OS distribution; or (3) use the archive . See this post in Stack Exchange network and please also perform a web search such as PHP Ondrej Archive to find the archive resource you really need

One of the links will be

https://launchpad.net/~tomvlk/+archive/ubuntu/php-archive

Please note that it's not any kind of official repository so use it at the your own risk

1. un-commenting will not work on Ubuntu/Debian libux as you should install extension like sudo apt install php8.2-curl

2. i believe you do not have php-fpm installed with apache and using default .... hence you need to restart apache2 for changes to take effect. you can check if php-fpm is configured with apache either via phpinfo() or via sudo service php8.2-fpm status .

you may refer this article for detailed information on how to install. Please also note you might have to add Ondřej Surý’s PHP PPA to get latest stable versions of php for ubuntu