```
function foo()
{ echo 'function called'; }

```

Is it possible to do something like this:

```
onchange($a, foo);
$a = "foo"; // echoes 'function called'
$a = "bar"; // echoes 'function called'

```

Instead of:

```
$a = "foo";
foo();      // echoes 'function called'
$a = "bar";
foo();      // echoes 'function called'

```