Actually, this can be used in another case, to test for errors:

do {
    if(!test1()) break;
    if(!test2()) break;
    if(!test3()) break;
    /* actual code */
} while(0);

This is pretty useless in C, since this sort of thing is much better handled by goto (yes, yes, I know, but even the Linux kernel does it). And higher level languages can use exceptions. But in PHP, before version 5, this was the best way to do error testing and defensive programming.