PHP plugin general problem

Hi,
I have a general problem using any of the following PHP plugins: EXEC PHP, PHP Exec, PHP Execution, WP Include File.
All of them, instead of displaying the correct content is showing the PHP code.
It seems that the PHP code is not interpreted, for example when I use in the post the following PHP instruction “<?php echo “This is the Exec-PHP Hello World”; ?>” I get “<?php echo “This is the Exec-PHP Hello World”; ?>” as a text instead of “This is the Exec-PHP Hello World”.
Any idea of what is going wrong?