PHP using Declare ? What is a tick?

I am a little bit confused by the PHP function declare.

What exactly is a single tick? I thought a tick equals one line of code?

But if I use:

function myfunc() {
        print "Tick";   
}

register_tick_function("myfunc");

declare(ticks=1) {
   echo 'foo!bar';
}

The script prints:

"Tick" 2 Times??