Is is possible for a php script to know if another script calls it via require or require\_once?

eg: 

```
if scripta.php calls me do xyz;
if scriptb.php calls me do abc;

```

*Edit: Thanks for your suggestions guys. It was more of a what if question, rather than an actual problem. I realise I could set a variable, $caller and update it when I made the require statement. I just wondered if there was another way to do this in the file that's being called :)*