How to run some command before or after every Bash command entered from console?

I want to run a command, for example

echo "foobar";

After each command, entered by the user.

Two scenarios:

• When the user enters a command, my global command should be executed, and later his command should be executed
• When the user enters a command, his command should be executed, and later my global command should be executed

How to accomplish the above two scenarios?

NB: I don't want to use the prompt for this purpose, (leave the PS1 variable as is).