I'm using Apache on Ubuntu, and each PHP page shows nothing. However, other files, for example images are displayed normally. What must i do?

Begin by altering the file extension of your php files to .php4 or .php5.

Should you still visit a blank page you may have errors inside your code, to debug place the next code towards the top of your php files:

error_reporting(E_ALL);  // will report any errors your code may have
ini_set("display_errors", 1); // will display those errors

Still visiting a blank page ? If you are on hosting that is shared please disregard the following step and report the issue for your host company.

It is possible that php is not installed properly with Apache, to set up it on Ubuntu run:

sudo apt-get install php5
sudo apt-get install libapache2-mod-php5
sudo /etc/init.d/apache2 restart

Please Tell me whether it labored.