Try running the following at the command line.

To just get the version information:

php -v

Or to get a lot of info:

php -i

It should give you all information you need about the php install.

I use the following command to view installed PHP versions in Ubuntu:

sudo update-alternatives --list php

Second way go to php directory where all PHP version configuration file stored:

cd /etc/php 
dir 

Output:

 > 5.6  7.0  7.1

Do it from your command line:

php -v
mysql -V

and:

php -i | grep -i '^libxml'

OR

Put this in your root directory:

 <?php
    phpinfo();
 php?>

Save it as phpinfo.php and point your browser to it (this could be http://localhost/phpinfo.php)

Put this in your root directory:

<?php
phpinfo();
?>

Save it as phpinfo.php and point your browser to it (this could be http://localhost/phpinfo.php)

• More information (you can get much more information than just the version).
• Example (random image):

The PHP module used by your Apache server (and thus, the one shown "in the browser") is set in Apache configuration.

Depending on how you installed them, you can try something like

a2dismod php7.4
a2enmod php8.1

this will disable your old 7.4 module and enable the 8.1 version. You then have to reload Apache for the changes to take.

If it doesn't work, just invert the commands to re-enable your 7.4:

a2dismod php8.1
a2enmod php7.4

Be aware though, that depending on how you installed the modules (from source? from a .deb package?) the above might not work. If you can give more information about your Ubuntu version and how you installed the extra PHP versions, that'd help give more precise answers.

On Debian (and thus Ubuntu), the Apache configuration files are stored under /etc/apache2. In that directory, there are 2 sub-directories for configuring modules: mods-available and mods-enabled. When you install an Apache module (ie: foo), it will put foo.load (and possibly foo.conf) into the /etc/apache2/mods-available directory. When you enable an Apache module using a2enmod, it will create a symbolic link in /etc/apache2/mods-enabled for each of the matching files from /etc/apache2/mods-available.

Thus for an Ubuntu server with PHP5 enabled, you should see something like this:

$ cd /etc/apache2
$ ls -l mods-*/*php*
-rw-r--r-- 1 root root 133 2008-02-27 15:49 mods-available/php5.conf
-rw-r--r-- 1 root root  59 2008-02-27 15:49 mods-available/php5.load
lrwxrwxrwx 1 root root  27 2009-02-05 07:30 mods-enabled/php5.conf -> ../mods-available/php5.conf
lrwxrwxrwx 1 root root  27 2009-02-05 07:30 mods-enabled/php5.load -> ../mods-available/php5.load

If the php5 configuration files aren't shown in the mods-enabled directory, enable them as follows:

$ sudo a2enmod php5
$ sudo /etc/init.d/apache2 restart

Once you've done that, in order to test that PHP5 is configured, create /var/www/test.php as follows:

$ echo "<?php phpinfo(); ?>" | sudo tee /var/www/test.php
$ sudo chown www-data:www-data /var/www/test.php
$ sudo chmod 755 /var/www/test.php

Once that's done, you should be able to browse to /test.php on that server and see the PHP configuration data.

This command works while running in PHP

<?php
    echo PHP_VERSION;

You can get it in bash, like

PHP_VERSION=$(php -r "echo PHP_VERSION;")

Here is all of PHP Predefined Constants